Joseph, you are spot on with the flex and weight issue off the spare. I haul bikes off a spare tire on an old landcruiser, but the spare rack is over built, and even so, it takes a lot of abuse.
Yeah my bikes are old, ugly, and heavy, just like me!
I doubt PC over built the spare holder, and we already have 17's, which adds more weight yet.
I've been looking at a hitch rack by 1up, maybe even doing a front bumper mount if I replace the front bumper. Although I'm not a big fan of blocking intake air, I think it would get them more out of the way than on the rear.
I've got a few ideas for the rear too, Just don't want to look like the Clampets....
Doug