Yes, but by the very nature of the ad, only posting members will get their sites advertised, and, the more they post, the more exposure their ad gets. And since we have a very active moderating community here, it's unlikely that a spammer will go very far.
I don't mind ads in sigs in particular, because I think it is rewarded by good behaviour and regular activity. I'm not neccesarily talking about "buy-viagra" ads, but, say, ads advertising another forum, for instance, or just a personal website -- relatively harmless ones.
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