A very interesting topic indeed, which is always true when Charles creates a topic.
Lets see what I have ... a 6" rigid for PC, a 6" flexible for PC, a 5" rigid for PC, a 4" rigid for PC, a 6" flexible for rotary and 4" rigid for rotary.
I just recently got the flexible BP for the PC and while it does work, I don't find it to be all that great. There is something with the oscillation and spin of the pads that doesn't seem to work like a rigid one. I actually had a pad fly off my UDM with the flexible BP because the backing plate flexed on a fender and the pad didn't - the speed and angle was great enough to rip the pad off of the backing plate. This has never happened to me with a rigid backing plate.
With a rotary, I've only used a flexible backing plate, with larger pads. I haven't had a single issue, unline with the UDM. However, after visiting with Aaron from Edge, I have been considering switching over to the Edge pads and adaptors for rotary use. The perfect centering, quick disconnect, double sided pads are looking good.
I doubt I'll switch to Edge with a my PC and UDM for a few reasons- I have 20-30 hook and loop pads, and Edge pads haven't held up 100% with DAs (nothing against Edge, the DAs are just very brutal on glue holding the pads to the plastic disc).
Also, Jason Rose from Meguiar's made an interesting point at the Expo. He only uses the backing plate and pads suggested by the manufacturer. If he needs to use a 4 inch pad, he switches machines to a 4" pneumatic machine. He says the companies have tested and harmonized the machines to work only with the MFG recomendations. Now many, myself included, switch between 4" to 6.5+" without issue, but it may be another thing to think about.