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.
You do forget things you are not working on. Nothing can be done about it. I could read German easily by the end of 8th grade and now I can hardly spell "Entshuldigen Sie mir bitte". There are several math. papers I read as a student of which I remember next to nothing. The most frustrating and shameful thing is that I don't remember the details of my own papers written 20 years ago with a few exceptions. After age 40 I also started to lose the ability I always took for granted: to get to the board at any time and start lecturing on some subject in my field with full proofs without any preparation. Now I have to sit for half an hour and to prepare my lectures now and then (thanks God this concerns only advanced graduate courses yet). And I work as a professional mathematician in academia full time!
The only way to cope with this loss of memory I know is to do some reading on systematic basis. Of course, if you read one paper in algebraic geometry (or whatever else) a month (or even two months), you may not remember the exact content of all of them by the end of the year but, since all mathematicians in one field use pretty much the same tricks and draw from pretty much the same general knowledge, you'll keep the core things in your memory no matter what you read (provided it is not patented junk, of course) and this is about as much as you can hope for.
Relating abstract things to "real life stuff" (and vice versa) is automatic when you work as a mathematician. For me, the proof of the Chacon-Ornstein ergodic theorem is just a sandpile moving over a pit with the sand falling down after every shift. I often tell my students that every individual term in the sequence doesn't matter at all for the limit but somehow together they determine it like no individual human is of any real importance while together they keep this civilization running, etc. No special effort is needed here and, moreover, if the analogy is not natural but contrived, it'll not be helpful or memorable. The standard mnemonic techniques are pretty useless in math. IMHO (the famous "foil" rule for the multiplication of sums of two terms is inferior to the natural "pair each term in the first sum with each term in the second sum" and to the picture of a rectangle tiled with smaller rectangles, though, of course, the foil rule sounds way more sexy).
Since it is a "general" question, I suggest making it community wiki (and mark my answer as such).
Best Answer
Well, a personal anecdote, I worked with a famous guy for some years. His basic strategy was to mail, on paper, anything publishable in a draft to one or two dozen parties that might be presumed to be interested. If nobody replied inside a month he submitted it somewhere. The point, in my mind, was that if one other person sees your stuff early you may get robbed, but if 20 see it early they are all witnesses. Later they came up with the arXiv.
The other, well-known side, is that if you share your stuff with the top expert in the field, that person may send you back a note saying "that was fun, here is the answer" and promptly forget all about it. You have not been cheated but there is still a problem.
EDIT There seem to be mixed impressions of what I meant in the preceding paragraph, and for whom the situation would remain a problem, so maybe I had better describe my own experience again. I have told this story many times, with names, and I think the story only does people credit, but I think on MO I ought to stick to anonymity. Email me if you want more detail. In graduate school I was working on minimal submanifolds. My adviser came up with a fairly specific problem, suggested I work on it, and asked one or two guys in the same department if they thought it was new, which they did. It still took me some time but I was getting there. My adviser was away somewhere giving a talk, and, once again, mentioned the problem to a guy. The difference was that this guy is a leading light in similar problems, went home, solved my dissertation problem in one evening on some 30 pages of notes, and put those in a drawer and forgot all about it. But at some point he happened to mention to my adviser that it was a good problem, he had completely solved it. My adviser mentioned this to me, and I was terrified. How could I submit this as a dissertation if this other guy solved it already? At some point I contacted him, he said, don't worry, it's your problem, I don't need it, you just finish it up and it's your dissertation. Finally, after I finished, I did ask to see his notes, he found them eventually and sent me copies, but even between him and his adviser at the 1992 Park City summer program no sense could be made of the notes by anyone concerned.
So I suppose I would say, along with Kevin's comment, that the nature and severity of the "problem" when the world champion in your area solves your problem in an afternoon (but has not the slightest intent to publish, ever) depends on your position and how much you need this as a publication/dissertation and how critical it may be that the work be perceived as your own and original. I may have misunderstood my position in graduate school, and everybody behaved well in my opinion, but it was certainly scary. I think I do see Kevin's point that, as a journal referee, he is often confronted by work that is already known, "in the air" as they say, or where the most likely techniques are pretty obvious as soon as the statement of the theorem is read, but he will still accept it for many journals.
I think it is fair to say people pick and choose what of their stuff to put on MO. This is probably healthy. We should struggle rather than getting handed everything.
Given that this question is Country and Western, I am taking this opportunity to point out that I went to high school with Paul Ginsparg, founder of the arXiv. He was a year older. It is a good bet that he is still a year older. Also Natalie Portman.
http://en.wikipedia.org/wiki/Syosset_High_School
http://en.wikipedia.org/wiki/Paul_Ginsparg
http://en.wikipedia.org/wiki/Natalie_Portman