iOS will jump up sharply from 0.4% in 2011 if the folks at Stack Overflow make the site usable with that system. From mobile Safari the site is crippled. You can type in an answer, but the formatting tools and preview panes don't work at all.
While I appreciate there are a few hardcores who will want to answer questions on the go, I can't see that being a big priority.
I answer less questions than I normally would if the interface were better.
(maybe this is fitting into the definition of "a few hardcores" for you though)
You're also focused on the users of SO specifically. There are Area 51 sites that would be more prone to having users on mobile devices.