I like not having an avatar because I like not being remembered. If people start to recognize me and remember things about me, I feel like my ability to have an honest conversation with them is threatened.
I know this sounds really creepy and antisocial, but it's actually more like a long experiment I'm running. I'm open with my personality on other sites, and people treat me very differently there. Sometimes I just want to be another blank face in the crowd.
(Although if not having an avatar becomes very rare, I'll definitely use Grumpy Cat.)