I would throw in here the analogy of signatures. The Mods have to enforce the TOS which says no commercial signatures. That is the only enforceable TOS for signatures, yet signatures still cause plenty of problems, complaints and feelings, despite the fact that users can elect to turn off signatures. Spammers are getting much more subtle - we've had some real issues with innocuous looking signatures, but which were really just nothing more than spam that the member was freely posting around the board because it was in his signature!

We get complaints from members about what are said in signatures or how irritating some members find them, despite the fact that members don't have to view signatures. And of course, then there are the members who chose to send messages in their signatures...

And that's just from a couple of lines of text - they do say a picture paints a thousand words!

So when it's argued that avatars won't cause any problems - I look at the problems which signatures cause and just laugh. But of course, unless you are or have been a mod, you are generally blissfully unaware of all this additional work going on behind the scenes to ensure that your viewing experience is as enjoyable as possible.