That's the main reason I don't have a paid signature. It increases your probability of being banned significantly!
I dont think thats true. The signature does not the ban make, the post quality is what does it. The correlation is between signature advertisers and low post quality, not so much bans to signature advertisers. While having a big flashy advertisement does make your spam stick out more to moderators, if you aren't spamming then it doesn't matter.
Actually I think that you're wrong this time. I actually had a screenshot of some staff member saying something alike. Anyhow, when you post a lot without a signature you get eventually noticed and if it is really bad you get banned.
Now, if you have a signature you're like a target. We rarely see someone get banned for making 15 posts a day (decent ones) without a signature do we?
We don't go searching out people who have signatures, people are just more prone to report people with flashy advertisement signatures as their spam is more noticable. How it works: someone reports a person's post in a thread for being off topic/spam whatever. We then read the thread to see if it is indeed off topic/insubstantial/against the rules if that is needed (Ie not a +1 or smiley face post) then we check that user's post history to see if they have a history of insubstantial posts. If not, the post is deleted and we move on. If they do, then the moderator posts the ban request and reason with evidence for Badbear/Theymos to review (As only Admins can temporary ban people (thank's SMF!)) it is then reviewed by an admin who does the banning.
Something to note, the only people the public really see get banned are those who make threads about it in meta. With that, the new trend of spamming in Meta as moderators are more careful about removing posts in Meta, giving the people "appealing their spam bans" the ability to spam posts while they are banned. I'd agree with the fact that there are more people with paid advertisement signatures that are banned daily than those without, but I could probably argue that they have more motivation to break the rules than anyone else, and people with paid sigs also make up a very large portion of the community.
My point being, if you have never been banned, you then get a paid advertising signature and continue posting how you have in the past, you aren't going to get a ban just because of the advertising signature. If you let your post quality deteriorate to meet quotas as tends to happen, then you do risk being banned.