To my knowledge, Contributor uses the same infrastructure and technology as ads, but with a different payload. All 3 problems you mentioned are problems being experienced today:
1. Google is already able to guage 'low quality' pages, co temt farms and pirates. This doesn't change
2. This happenens with ads, Google can add a rule saying you "can't have x amount of ads of you want to be on our network "
3.Likely to stay the same: I don't see how contributor makes fraud easier
Absolutely, those are problem we know exist and this model risks making worse. The basic arguments is the big move towards cost by impressions if this catches on and becomes a dominant model, and the fact that banners of limited visibility is easier to stack. Huge white, or even invisible, banners littering the page will not disturb the casual user.
Google lost the fight on content farms several years ago. You are more likely to find reprints than original material if you google a breaking story or meme for example, even if there are exceptions.
1. Google is already able to guage 'low quality' pages, co temt farms and pirates. This doesn't change
2. This happenens with ads, Google can add a rule saying you "can't have x amount of ads of you want to be on our network "
3.Likely to stay the same: I don't see how contributor makes fraud easier