Proof of click is a very good idea but there are some technical limitations. It's very hard to track ad clicks because of steem "privacy/security settings".
But they solved the problem good by giving upvotes as a compensation for clicks that may be fraudulent. Let's say you spend 5 sbd on ads. Then you get 5 sbd advertising budget + they will upvote your posts for 5 sbd worth - curation rewards. So the only cost of your ads are the curation costs. That's a good deal.
As a content provider i think it's also interesting because you can get extra revenue when people click on the dclick ads on your post apart from the normal post payouts.