We indeed had a lengthy discussion about this on IRC yesterday, and there are definitely some interesting things to consider. Fact is though that SourceBans has had a public ban appeal/request system for 6 years, and I haven't heard a single complaint of abuse. So for me it would be more about convenience (people not knowing or having to look up their Steam ID) than protection. And I agree that requiring them to register and remember yet another username and password would most likely scare people away, so using Steam login would be a nice middle ground.