A few people who are (overly, in my opinion) concerned about their privacy and security don't have Javascript enabled in their browser, so stuff that requires it wont work.
Thats the only real reason I can see for trying to do as much in CSS3 as possible other than just for the challenge.
Turning Javascript off (or at least manually whitelisting via NoScript) also can give a quite big performance boost, by simply turning off many useless effects.
Making CPU-hungry CSS3 kind of defeats that purpose, of course.
Thats the only real reason I can see for trying to do as much in CSS3 as possible other than just for the challenge.