I honestly think what happens is that the more units you return, the more you inspect the next one you get. I had this happen with iPhone 4 and iPad 2. Dead pixels must have been a lot more common back then, because I had them on both of those items more than once. Apple was really patient about letting me exchange for new units, but every time I exchanged, I got more and more nitpicky about other things--most of them being miniscule cosmetic issues that nobody would ever notice. It's a viscious cycle sometimes.
I will never put up with dead pixels, EVER--but I haven't had any on any of my Apple products since the iPhone 4/iPad 2 days. I do think it gets a little crazy around here when people are fretting about screen temperature and stuff like that and trying to figure out which display maker's screen is inside their iPhone/iPad/Mac. The conspiracy theories on Macrumors get pretty crazy--I wonder what some of the Apple Store employees think when people bring their stuff in and say "I want one with an LG display. This one is a Sharp. I want a cooler display temperature."
I do understand the need to have my Apple stuff be as close to perfect as possible though--it's an illness.