It seems to me that most of the objection to avatars is anti-social in nature, or at least suggesting that they don't add anything. What they add is that a real human being is behind what they typed. There is something powerful in that. We're not robots speaking to each other -- we're real people. And avatars lend to this social aspect. I'm not talking about socializing as much as I'm speaking natural human social interaction.
As for the technical side of it, I've found that most people don't even bother uploading avatars. And for those who do, you can enforce maximum dimensions and file size. It's not a huge bandwidth increase -- it's manageable. As for the moderation aspect of that, I think that's overblown. I've never seen anyone upload an avatar that was objectionable. I'm not saying that would never happen, but I don't think it's something to worry about.