A strong disbelief in the Honor System working in a community this large.

I've nuked quite a lot of spam in my tenure as a moderator, and I'm just one of a small volunteer group that often has a lot of ground to cover. Many FlyerTalkers do not see anywhere near the volume of spam that gets nuked here because we are pretty good at what we do.

If avatars are enabled, then not only do the moderators have to watch for text-based spam, we also now have to look for image-based spam or inappropriate images from spammers.

So, I'm sorry, but I feel quite strongly that adding graphics, in addition to causing more overhead to the IBB system that runs FlyerTalk, will very likely cause an increase in the workload for those who volunteer to help make the FlyerTalk experience enjoyable for all. Whether that workload increase would be too much or negligible compared to increased/decreased user enjoyment is another matter. You asked me on what I base my opinion that avatars will increase the burden of moderation. There's your answer.