I'm surprised that no one has yet to bring up an important issue here, and that is that, simply put, the 36x56 back is still not a full size 645 sensor, and will probably NOT be embraced by the commercial photographers who can actually afford one. And this, of course, is presuming that the sensor's true measurements are actually 36x56mm.
My reason being that the 36x56 back is SHORT on the 36mm side, yielding a 2:3 image ratio, rather than the preferred 3:4 ratio used by so many commercial & portrait studios. And if you crop the 36x56 back to a 3:4 ratio, you're back to a 36x48 again.
Landscape photographers would certainly embrace it, as 2:3 horizontal images have so much eye appeal. But then again, how many landscape photographers will be able to afford it? Maybe George Lepp, if he ever tires of Canon's endless support and endorsements.
In the end, I think these manufacturers figured out that it is easier, from a production-cost point of view to take a 36x48 sensor, and stretch it out to 56mm on the long side, but that stretching it in both directions, say to a 42x56 (which is what is needed for a true 3:4 image ratio), is proving to be a lot more difficult. I don't see that we're any closer to a 56x56 square back, and I'm not sure that there would be much demand for it, even if the technology were in place.
When it comes to rectangle images, I prefer the 2:3 ratio for horizontal images, but prefer the 3:4 ratio for verticals. A square image certainly eliminates all these issues, decisions, and choices about rectangle ratios, and whether to shoot horizontal or vertical.
I mean, after all, there's only ONE square ratio!!