Well, for those of us who think more visually than textually, they do help with recognition of individuals.

I'd support the inclusion of avatars in users' profiles, and possibly attached to comments in explicitly social subreddits/forums/etc that may be formed in the future, but I don't want to see them on the main site, mostly because I don't want the layout to be changed to accommodate that level of clutter.

ETA: Also, having avatars on the main site seems likely to contribute to priming issues.