Yeah, it kind of serves no purpose, really.
You run the risk of it being seen as gross and then possibly losing avatar priviledges, and it's pretty obvious from this topic that it's not going to go down well.
I wouldn't recommend using it as an avatar.
Even if people have had it in their signatures before. That's not the same as an avatar. Signatures aren't plastered on the forums anytime you post. Someone has to choose to visit your profile to see that.
And I think we all know that the forums don't operate like a court. Precedents don't really matter. We have rules set out, but how they're administered and if any other unwritten rules are going to be enforced is entirely up to the moderators.