Paid signature campaigns are a tremendous part of this forum as regards traffic, we cannot argue with the fact that removing signature campaigns would definitely affect the forum traffic, i don't mean it would run it to the ground, but truth be told, many users wouldn't post quite frequently, and it is fine/normal/acceptable. There is nothing wrong with paid signature campaigns and suggesting to remove it to combat spam is quite too much to ask, spammers hardly ever make it into signature campaigns because they can't rank up due to the merit system, they are in one bounty campaign or the other, thus it is futile hindering the activity of good users who earn from the forum through their good posts, it might combat spam but it would be a pyrrhic victory.
I don't think any mistake about paid signature campaign and have little fault about participants post likely have spam, there are have way keep report their low quality post based on your viewed and I don't think here with many spam post make signature campaign is the mistake made many people not care about their quality post.
Based on personal habit about post quality having and depending which viewed from some one, maybe when user A having bad opinion with low quality post, I think have different viewed by the other with not low quality post. Based on who see and have opinion about some one post could said with low quality or not.