I guess it depends on how you define a "RB who can catch." For Mornhinweg's system, I think you're talking about more than just a RB who can catch dumpoffs. I think you need a RB who can run routes and catch passes downfield, like a WR....and those RB's are rare. How many of those guys are there in the league?
I don't think that they're that rare. I haven't seen an RBs run the full route tree. Usually, they run slants, digs and maybe a fade route close to the LOS. The way I see it, the Pats seem to have 2 or 3 of these guys on the roster every year. No they're not as talented as the ProBowlers you mentioned but, again, no one on this offense is as talented as most ProBowlers (aside from Mangold).