This has already become a tradition of the forum. If the user is accepted in ChipMixer, then expect a topic from him soon about renting his avatar.
It wouldn't have become a tradition if other campaigns didn't require an avatar change. I get that the signature can satisfy for an advertising area, but doin' that for the avatar too “breaks” the forum's principles. On every forum that I've been on, each user either leaves empty or changes his avatar to something unique. Something that represents him, placing to your memory that this image is from that user.
I wouldn't be renting my avatar, but since this uniqueness misses from bitcointalk, I don't have any reasons not to proceed into that decision.