I bet one of the reasons behind the link units is that by providing a long list of advertisers, it's a good cue as to which ad gets picked out of the crowd. And on that basis they can detect a high CTR ad?
You don't think they can detect a high CTR ad simply by normal AdWords (search results at Google) placement?
Yes, but it skews toward the high paying ads. I mean for testing the low bid ads. Sorry I didn't explain better.