Why do you want a medium format in the first place? What's your goal? Adding a new format to your current line up is a bit of commitment. You'll need different reels and enlarger lens, etc, not to mention a whole new set of lenses and accessories for your new gear. Then there is a choice of 645, 6x6, 6x7, and for RB, you can even go 6x8 with later bodies. I know this thread started with "found an RB on C/G for $200!" but is the price a prime motivator or do you have something specific in mind? RB and RZ are seriously heavy camera for hand-holding use. 645 is a lot easier. I don't want to further confuse you but I think this is an important consideration - your goal.
Well, when you put it that way, why would any of use need anything other than 1 camera and 1 lens? :-)
You bring up a good point though. I'm definitely going to give that some thought before I make any decisions.