In fact most of the signature campaigns now don't count low quality posts. So the signature spams also won't be counted. I hope that people can think before they post.
Are you sure about that? I think managers do check the posts but I don't think all managers check all posts and campaigns using bots will count spams and it may not count substantial posts. Most bots check the number of words in user's post which is wrong to check whether a post is substantial or spam. Some spams may look like substantial post. It is hard to know "real" intention of the user. I do think signature campaigns increase spams.
Simple the signature campaigns maintainers should accept only valid users in their sig. campaign if the allow each user to enroll it is normal that the spam will increase (but as someone said yesterday (a well know term term) "any publicity is a good publicity") because more posts >> more view of the signature but when you try to write more the quality of the posts will drop.
It's not only about publicity, but also about SEO. The more links to a site, the better it will get ranked. That is, at least as far as I know.