There have been discussions on several of the newer SE sites on adding MathJax where the decision of the SE team leaned towards not enabling it on those sites. Sites like Cognitive Sciences or Biology currently don't use a lot of math, so the MathJax dependency was deemed as too heavy for those sites.
While I agree that it's difficult to justify a significant cost to every pageload when only a few posts actually use that specific feature, I think that all scientific SE sites should have MathJax available. Mathematics play an important role in all scientific disciplines, and not enabling MathJax might discourage questions and answers with significant mathematical content.
From a naive point of view without knowing about the internals of the SE engine my simple solution would be to only load the MathJax library on pages that actually use it. This would allow us to use MathJax even on sites that have a majority of non-mathematical content.
I also think that MathJax support would not be necessary in titles and excerpts for sites like Biology. It would only be necessary on the full question pages.
The new MathJax 2.0 also seems to be pretty modular (See also this recent post of MathJax developer Davide Cervone), which might reduce the impact of adding MathJax to sites that don't use Math heavily.