Ive only seen MUPs in DC and in the SF Bay area. I've lived a bunch of places, but mostly rural areas. Perhaps my opinions don't match up with many, but they are based just on the local rules for those two places I've lived. There, the general consensus is MUPs are for kids riding their bikes, families, etc. Commuters and roadies are generally regarded as belonging on the road if available, and to be very careful and courteous on the paths if they ride on them.
Where I live now in the SF area plenty of commuters and roadies ride on them, and we have hundreds of miles of them here, but the speed limit is 15 and it is well understood that everyone else has the right of way.