If you can carry a monopod, most of the shutter speed issues are negated too.
And the back problems! =)
Just had to chime in hear one more time-- While I'm a big 645 fan, and 645 can be made to have the conveniences of 35mm, it is frequently with quite a large weight penalty. A stripped 645 or 6x6 camera is a beautiful thing, but once you want the convenience of AE, a motor drive, and zooms, while it can be done, is not exactly easy on the back. Not sure if I clarified that in my earlier post.
I tried out one of the Schneider zooms for my ETRS a while back... that, paired with a motor drive (had access to the older one, not the nice, light EII) and an AE prism was a massive and heavy beast. Even having the upper body of a rock climber, it wasn't exactly comfortable or easy to raise to my eye. Probably at least 10x the weight of my little Pentax ZX-5 with a 75-150 zoom mounted on it.
So, to addend to my earlier statements, 645 is a fantastic all around system *especially if you don't need the conveniences of a motor drive, zoom lenses, and very accurate auto exposure*. Your back will thank you.
If I did need those, I'd probably still be on Nikon, with an F5 or F6.
For me, taking about 25-30 frames on a big day (MF), and shooting nothing but primes in 135 and MF, it makes sense to me =)