Call me biased as I am wearing one, but I don't think a paid signature is really the problem. I see (new) members spamming low quality posts without the use of signatures about as often as I see low quality posts from members wearing a signature ad.
While I am not trying to bash the moderation of this forum at all; all of the staff do a great job keeping the forum clean, I think there should be perhaps some better defined guidelines against what is and isn't a spam post and strict moderation on said posts.
Paid signatures do incentivize spam/low quality posting, but I don't believe that they are the overall root of the problem.