I think this thread is the perfect example why paid per post won't work large scale. If you look at it there are 4 pages of not a whole lot. Sure some from each side.
But does this thread bring knowledge that others will benefit from? Not really. Should it be locked at this point instead of going on and on? Chances are yes.
But that is just my opinion. I still think it's better with smaller campaigns then full forum paid per post.
I agree with your point. An easy solution would be to employ people to check posts one by one and use pay per rank instead of post. I have seen various people posting pointlessly and for example, spammers can just makes comments like "this is a great tutorial , I have made lots of money thanks!" And would still be paid for the post. The main reason for this would be because the signature campaign is operated using a bot to check post or signature owners just looks at the topic and judges the relevance of the post. No matter how many people claims that the "tutorial" doesn't work or is a scam. The poster could have just elaborated on the point instead of just posting that it "works".