I'll just chime in to say that I DID see Memphis and for me it was just mediocre. And my recollection of how it was received by theatre fans, and in my friend circles, matches yours. Sometimes at parties, or at the office (we're veering into major nerd territory here), we'll see if we can remember all the best musical winners by year without looking anything up and I have seen on numerous occasions that people can't even remember it won the year it won. I also agree it's one of the weakest shows to ever win best musical. Context matters as you've said.
All that said, Memphis is not something I disliked. It just left virtually no impression on me, except that is seemed like a melting pot of other shows that I thought covered similar ground much better (Dreamgirls and Hairspray come to mind). Wayman has been a staunch advocate of Memphis from day one--I was predicting how he would respond to your post. :-). And more power to him. I say, without sarcasm, that it's wonderful to have a show that one feels passionately about and wants to get behind.