Originally posted by Zygonyx Canikony supporters see the point because actually you may not get thinner depht of field with the P645 system than with any of their very fast f/1.2 and even f/1.4 primes.
If they really wanted shallow DOF they would all be using Leicas with the Noctilux 50mm f/0.95. Most Canon/Nikon photographers want the easy, take anywhere convenience of 35mm, medium format requires more of a commitment than 35mm. In terms of technique Medium format is harder to use, there is a higher cost of entry and lenses, filters, tripods and ball heads all cost more because of the increased physical demands placed on them. There is more to Medium format than the superficial needs of shallow DOF, you're much better off using 35mm where the lenses are still* smaller than the Medium format equivalents would be.
The fastest Medium format lens ever made was an 85mm f/1.8 that covered full 645 format - and trust me, that is as fast as you would ever want to get for medium format.
* The Carl Zeiss OTUS 55mm f/1.4 takes the prize for the biggest 55mm lens ever made for the FX format. The Pentax D-FA55mm f/2.8 is smaller than the OTUS.