I think this would be nice to have, maybe on a per-tag basis, but turning it on is definitely the first step.
Because it is disabled site-wide, we can't force it by manually specifying the language as described here.
Caution: This vote is not about wether or not we should enable it automatically when a certain tag is used, it's wether or not we should enable code highlighting on the site.
Upvote the according answer below.
Update: I think it's pretty clear where this is going. Can anyone do this change?