I understand it being fine to not have an avatar if there is a default avatar on the forum. As that way the post layout is basically the same with every post so there isn't really an issue.
But when there is no default avatar I can really understand the annoyance a forum owner would have.
But can't a forum owner just add a default avatar to those who don't have one set if it really is a bother?
I mean avatars are great but sometimes people don't have the time to put one up and instead just want to post.
I have suggested Gravatar to a few forums before. I feel that's a great integration service for forums.