If signature campaigns have been running for years and are still running, it clearly shows that these signature campaigns are profitable for marketers to run and they do get clicks. Posts on this forum can be found on Google, and with some people having thousands and thousands of posts, it's very likely even someone browsing Google for information that somehow lands on Bitcointalk will click on a signature after getting the information they need, as one change in avatar and signature means the signature and avatar is changed on all of your posts.