So, you need the MP. But the medium-format cameras suck compared to dSLRs from Canon/Nikon in everything except the MP count.
Medium Format cameras suck now?
DXOmark scores (I don't think they've tested the Leica S2 yet):
PhaseOne IQ180: 26.5bits, P65+: 26.0bits, P40+: 25.3bits
Leaf Aptus 75S: 24.7, Hassy H3Dii 50: 24.7, Pentax 645D: 24.6, P45+: 24.2, H3Dii 39: 24.2
1Dsmk3: 24.0, Sony a77: 24.0
So 3x MF before the D3X, then 5 more before the 1Ds3 and Sony a77.
P40+: 13.0, P65+: 13.0, P45+: 12.9, H3Dii 50: 12.7, Pentax 645D: 12.6, H3Dii 39: 12.5, Leaf 75s: 12.5
a900: 12.3, a850: 12.2
So the D3X wins and Sony is 3rd. But still 8 more MF backs and 2 more Sonys before the 1Ds3.
ISO doesn't count because you're in the studio, and neither does FPS (you find me a flash that can recycle 12fps), and AF doesn't count either for sit-down portraits with narrow-ish aperture. Most MF cameras have leaf shutters, so have higher XSync speeds than 35mm. As for lenses, they may cost a lot more than the canikony types, but you get a lot of lens. And don't forget the AA filter on MF is a lot less intrusive, and the diffraction-rolloff aperture is a lot higher in number.
If MF backs/bodies suck, it's in the price tag, and the weight, although Nikon's D3X is giving them a run for their money (at not much cheaper than the Pentax 645D though). Canon's got a lot of catching up to do by the looks.