best practicesmath-mode
In my document, the numbers inside math mode appear differently than those numbers out. Sometimes, I have numbers like "10 squared" within a paragraph, so it seems useful to use $10^2$. However, maybe on the same line, I have "10 km". The style of the 10 is different. In this case, should I also use $10$ km?
\documentclass{article}
\usepackage{mathspec}
\setmainfont[Mapping=tex-text]{Linux Libertine O}
\setmathfont(Digits,Greek,Latin)[Numbers=OldStyle]{Linux Libertine O}
\begin{document}
$0123456789$
\end{document}
Two objections come to mind:
For example:
\documentclass{article}
\newcommand\mathmacro[1][A]{\ensuremath{{#1}_1}}
\begin{document}
\mathmacro[$x^2$] % The dollar signs *leave* math mode
\end{document}
What's going on here is that \mathmacro[$x^2$] expands to
\ensuremath{{$x^2$}_1}
which expands to (effectively)
\ifmmode
{$x^2$}_1
\else
${$x^2$}_1$
\fi
and you can see that if you write it outside of math mode, the second branch is taken, so the first dollar sign brings you into math mode and the one at the front of $x^2$ takes you out of it, with the reverse operation happening afterwards. This gives an error.
Of course, you aren't supposed to do that, since \mathmacro is actually a "math macro usable in text mode", so you should think of the thing between the brackets of its argument as being in math mode. Alas, this confuses both the author and the text editor's syntax highlighting, since it is a nonstandard assumption.
Edit: I would define this macro as:
\newcommand\mathmacro[1][A]{{#1}_1}
and use it as:
$x^2$ sub one: $\mathmacro[x^2]$.
This way, the parts that are math are clearly math.
\everymath, you will be very surprised when your apparently text-mode macros start to look different.On the subject of semantics, though, the issue is clear: \ensuremath breaks the separation between math and text, which are two very different things. TeX even has the distinction built in: different fonts, different spacing rules, different parsing rules. You can probably construct a lot more counterexamples by exploiting these.
What I mean by this is that in the following situation:
\documentclass{article}
\newcommand\mathmacro[1][A]{\ensuremath{{#1}_1}}
\begin{document}
A sub one: \mathmacro
\bfseries A sub one: \mathmacro
\end{document}
you may be surprised that the bold text does not extend to the contents of the apparently text-mode \mathmacro.
What I'm saying is not so much that \ensuremath actually breaks anything as that it violates your expectations to the point that it makes things harder rather than easier.
Best Answer
Don Knuth touched on this topic in his article for TUGboat -- "Typesetting Concrete Mathematics". His examples don't include units (for that, the siunitx package is a good choice, as already mentioned), but the method for determining what is math and what isn't is well illustrated otherwise.
(The article is set in Knuth's Concrete fonts, and shows some of the special techniques used in setting that book. Irrelevant for this question, but interesting nonetheless.)