I agree that there is a strong need for equations on StackOverflow.
I'd like to point out another major need for TeX support on StackOverflow: answering quantitative programming, vector computation, and machine learning questions. I understand there are some programmers on this site that believe everything can be expressed with code, but it's pretty much impossible to explain that the code is correct without math. Since machine learning questions are becoming much more common now, as well computer graphics questions, both of which require linear algebra and probability, I don't see why we should unnecessarily restrict ourselves. Another big use case: optimization problems such as linear programming and numerical approaches to solving equations.
I would say questions that marry the arts of mathematics and programming are ones that I especially enjoy answering, and I have quite a few answers that would have been much improved by being able to write math, instead of the ugly fixed-width hack. Here are some examples:
I have even more answers that haven't been accepted yet that also use math; I also have tons of answers where I decided to just not even try to explain with math. It sucks that every time I want to post a proper answer to a machine learning question, I end up waving my hands instead of writing something that will be useful for the OP and many others. Wouldn't things be so much better if I could actually contribute my knowledge?
UPDATE: We have the ability to run code snippets now in posts now but not write math? Come on, it's 2015 - whatever performance issue hindered this from being implemented in 2009 surely isn't an issue anymore.