Not to mention that JavaScript support would make sense in a lot of places. Input validation, visual editor, autosave-to-scratchpath while working on long posts, better chatterbox, etc.

I think the problem is the "We'll just add Javascript for this one feature .." slippery slope. Then we also have to build in the ability to gracefully degrade the behavior when the browser using the website doesn't or can't do Javascript. And then the pm-dev folks have to know enough Javascript to maintain that code, as well as the Perl that runs the site.