We sure can decrease post made at here too, but at least we won't get banned since it is on topic now.
It isn't allowed by forum rules to post these things to some specific sig topic about these things. ( at least everyone says it is off topic)
We need place to talk about this, without getting banned.
Doesn't matter if we get paid for these or not.

Some people might want to use their sig to advertise their own site or ref links.
And some people just simply don't want to advertise anything.
But if you don't mind of having advertisement in your signature I can't see why you wouldn't use it.