I somewhat disagree with some of the earlier answers on the "jobs applications impact". First, I think it makes absolutely no difference whether you have "submitted to Annals" or "preprint" on your CV. Everyone knows the acceptance chances and will ignore this line. Second, it is really important that all your papers are available on the arXiv or your personal web page. Often, the hiring committee can't judge the applicant's work, and will ask a local expert at the department to take a look at the papers and give an impartial opinion. If the papers are not available, the committee is forced to trust the applicant on their existence, a bad situation all around. Finally, except for the really top journals, having a paper published vs. having it still in a preprint form is of minor difference - if the local experts and/or reference letters are all saying that these recent papers are really good, that's sufficient. From that point of view, you should basically ignore the job application considerations, and always do what's best for the paper.
P.S. If I may make a suggestion, I think it's much more important to choose the right people to write reference letters than worrying about minor points in your CV. Especially now, in the mathjobs era of mass applications, it is the letters that really help people stand out from the crowd. So my advice would be to start thinking well in advance who can you ask for the letters, and learn how to better communicate your results (to them and everyone else).
This is a question of interest to most mathematicians who are research active and not slowly but surely knocking off important problems in their field at the rate of one per paper. (I think I could have ended the previous sentence at the word "active" without much affecting the meaning!)
I think the answer is ultimately quite personal: you are free to set your own standards as to how much of your work to publish. I myself understand the psychology both ways: on the one hand, math is usually long, hard work and when you finish off something, you want to record that accomplishment and receive some kind of "credit" for it. On the other hand, we want to display the best of what we have done, not the entirety. This position is well understood in the artistic and literary world: e.g. some authors spend years on works that they deem not ready to be released. Sometimes they literally destroy or throw away their work, and when they don't, their executors are faced with difficult ethical issues. (This is roaming off-topic, but I highly recommend Milan Kundera's book-length essay Testaments Betrayed, especially the part where he details the history of how after Kafka's death, his close friend Max Brod disobeyed Kafka's instructions and published a large amount of work that Kafka had specifically requested be destroyed. If Brod had done what he was told to do, the greater part of Kafka's Oeuvres -- e.g. The Trial, The Castle, Amerika -- would simply not exist to us. What does Kundera think of Brod's decision? He condemns it in the strongest possible terms!)
Another consideration is that publication of work is an effort in and of itself, to the extent that I would not say that anyone has a duty to do so, even after releasing it in some preprint form, as on the arxiv. A substandard work can be especially hard to publish in a "reasonable" journal. I have a friend who wrote a short note outlining the beginnings of a possible approach to a famous conjecture. She has high standards as to which journals are "reasonable", and rather than compromise much on this she determinedly resubmitted her paper time after time. And it worked -- eventually it got published somewhere pretty good, but I think she had four rejections first. I myself would probably not have the fortitude to resubmit a paper time after time to journals of roughly similar quality.
As you say, though, one advantage of formal publication is that the paper gets formal refereeing. Of course, the quality of this varies among journals, editors, referees and fields, but speaking as a number theorist / arithmetic geometer, most of my papers have gotten quite close readings (and required some revisions), to the extent that I have gained significant confidence in my work by going through this process. I have one paper -- my best paper, in fact! -- which I have rather mysteriously been unable to publish. It is nevertheless one of my most widely cited works, including by me (I have had little trouble publishing other, lesser papers which build on it), and it is a minor but nagging worry that a lot of people are using this work which has never received a referee's imprimatur. I will try again some day, but like I said, the battle takes something out of you.
Finally, you ask about how it looks for your career, which is a perfectly reasonable question to ask. I think young mathematicians might get the wrong idea: informal mathematical culture spends a lot of time sniping at people who publish "too many papers", especially those which seem similar to each other or are of uneven quality. Some wag (Rota?) once said that every mathematician judges herself by her best paper and judges every other mathematician by dividing his worst paper by the total number of papers he has published. But of course this is silly: we say this at dinner and over drinks, for whatever reasons (I think sour grapes must be a large part of it), but I have heard much, much less of this kind of talk when it comes to hiring and promotion discussions. On the contrary, very good mathematicians who have too few papers often get in a bit of trouble. As long as you are not "self plagiarizing" -- i.e., publishing the same results over and over again without admission -- I say that keeping an eye on the Least Publishable Unit is reasonable. Note that most journals also like shorter papers and sometimes themselves recommend splitting of content.
So, in summary, please do what you want! In your case, I see that you have on the order of ten other papers, so one more short paper which is in content not up there with your best work (I am going entirely on your description; I don't know enough about your area to judge the quality and haven't tried) is probably not going to make a big difference in your career. But it's not going to hurt it either: don't worry about that. So if in your heart you want this work to be published, go for it. If you can live without it, try that for a while and see how you feel later.
Best Answer
In my opinion, as a rule the supervisor should not be a co-author in the main paper taken from a student's thesis, even if he has contributed substantially to it, and even more so in the circumstances you suggest. The student needs to publish much more than the advisor does.
If the advisor him/herself is a junior person and has given a lot of help and a very good idea to the student, then I suppose that an exception might be reasonable. Also, a thesis project might spawn more than one paper, of course, in which case it's fine if the advisor is a co-author in some of them (always assuming that he has done much more than suggesting the initial idea).
Of course, it may happen that the student is weak, is given a good project, and needs to be guided step by step, so that at the end the advisor has contributed much more to the thesis than the student. Then a joint publication is in order. Such a student will most likely not pursue an academic career, so it does not really matter.
[Edit] Let me try to clarify my thought, and perhaps be less radical. What I am going to say applies to pure mathematics; I am very much aware that in other fields things may be completely different.
A good thesis project is one that is both interesting and feasible. Devising such a project in pure mathematics is hard; most beginning students, even very bright one, need guidance, particularly in countries, like Italy, where the PhD program is 3 years. A student has a lot to learn before getting to a level to understand and appreciate a research project; it is clear that a student in a short program does not have a lot a time for trying and failing (which is, of course, very educational, but also time-consuming). Now, some students come up with their own problems and solve them, but in my experience they are exceptions.
I consider it part of my job as an advisor to suggest a problem, or an area of investigation that can be profitably mined from the student. After that, I follow the student, teaching her (let's say she's a woman, purely to avoid the "him or her") whatever I can, trying to dissuade to pursue lines of work that seem barren, uninteresting or risky to me, and also giving ideas. Sometimes she will get stuck; and then I'll think about the problem, to see if there is a difficulty that seems unsurmountable, or if there is an approach that she can try. After some time of this, if she is good she will take off on her own, and understand the problem better than I do; then I will consider that I have done my job. When she writes the paper, I will not be a co-author, even if I have obviously contributed a lot to the project.
Of course, different students require very different levels of involvement; but in my experience, it is not necessary true that the best student are the ones needing less help. Also, a lot depends on the problem.
Now, some people tend to give students substantial parts of their research agenda; in this case the advisor is directly interested in making progress, gets more involved, and is more likely to be a co-author. This is another case in which joint authorship is perfectly reasonable. I would not want to conclude anything about a student from the fact that have published the main paper from their thesis with their advisor.