Pretty much as it sounds.
Certain cryptographic functions can be described mathematically. Is there a need for MathJax here?
I think so. Even simple things like complexity, average number of iterations to "crack" something (as in this question) and so on would benefit from MathJax.
Another question here on meta asks how we can attract strong cryptographers to the site. Believe me, anyone good at crypto is going to want some kind of TeX support.
Take a look at any crypto paper from the last 20 years; it's hard to discuss anything deep without math. (You can still discuss "which block cipher is most commonly used with IPSec" without math, of course.)
If LaTeX notation was readily available, I, for one, would use and possibly abuse it. Right now I use italics and the <sub>/<sup> tags, which is relatively inconvenient and forces me, in a way, to use such notations sparingly. Enabling MathJax or a similar tool may somehow shift the contents of my answers (and probably that of other people) toward the mathematically heavy.
This is not necessarily bad. But one should be aware that the question of MathJax is not entirely contained in the difficulty of maintaining MathJax or downloading it; its presence or absence has an influence on who will answer what. As @Fixee points out, easy LaTeX support is likely to attract researchers.
I think we need some formula support integrated into the site (e.g. more than simply uploading images). It does not necessarily have be MathJax, where the formulas are interpreted on each page load - some server-side solution which creates and saves the images on posting (or on first loading) should do fine (like what Wikipedia is using).
Although I would really enjoy latex support, I wanted to point out this question from PhilSE(beta). There are only three SE sites with MathJax support: physics, math, and statistical analysis.
Jeff Atwood has said in the past that supporting MathJax is extremely burdensome. For this reason, MathJax support has been declined over at StackOverflow.
Ultimately, I suppose it might depend on how much actual math is done here. I do not think it was the intention of CryptoSE to overlap much with MathSE, so truly mathematical questions should be asked over there. And for much of the rest, HTML and a little ascii creativity .
Finally, if that is not enough, then it is always possible to create Latex images and use them, perhaps using the widely-used site http://www.codecogs.com/latex/about.php.