I understand your point of view, and I even agree with your disposition to automatically deduct credibility points from users with rented sigs. The quality of the posts tend (in some cases) to drop significantly, because the person is only interested in QUANTITY posting, but sometimes lack QUALITY.
So, the idea in deducting credibility points can be a way to stimulate more quality from this situation (or at least to be a filter to protect you from wasting your time).
But, anyway, there are exceptons: I mean, users who rent their signature and work (hard) to keep quality... (Although I know I am not the best contributor to this forum, I try to keep some quality).
It is not just about quality vs quantity. By involvement in a sig ad campaign, you are announcing to the world that you view your very own reputation as worth nothing more than a few shekels.
A person's reputation is not what is in their signature line, it is how they act and what they say. Having a paid signature is nothing more then receiving money for advertising a good or a service. If you are against advertisements then you should be against this forum (that sells ads) and any source of media/news as they make almost all their money from advertisers.