Well I assume that if swapping the screen doesn't fix it, then it's not fixed, which means they'd have to keep trying. I'm not okay with having a defective screen right out of the box, but I feel I'd rather have to deal with one problem on one machine (and them working to get it right) rather than risk another machine which may have a new set of problems like the ones already outlined elsewhere. I have no intention of returning the iMac outright, i.e. I expect them to fix it however long it takes. But sending it back to Apple and waiting a few weeks for another one, with the risk that I may have to do it all over yet again, is outweighed (in my mind) by the relative convenience of having it repaired at the store (presumably a lot faster) to my satisfaction when I know that theis particular machine has no other problems at all.
Maybe I'm being foolish, and again, I'm not ecstatic about the out-of-the-box condition, but to me, replacing just the screen locally is less risky and more under my control than the uncertainty of the other option.