But would you take it over a 5Ds2 ? Or, better question, would you want to own a medium format back at all? A lot of the time these are just rented, while most people own their 35mm bodies.
Here's where I see the niche. A guy has a medium format film rig left over from back when before he moved most of his work over to 35mm DSLRs. He might rent a medium format digital back from time to time when he needs those big files, but operationally, it's just not even close to the conveniences of a modern DSLR. So, now he buys this reducer and uses his old lenses on the new body when he's after a certain look but doesn't feel like renting out a back. Properly design, he would keep all the conveniences of his DSLR (minus AF)
I think that an even more fun thing to do would be to use an astigmatic reducer to create a modern digital XPan out of a DSLR. Something that squeezes the frame horizontally, but otherwise leaves it in tact vertically. You then re-stretch it in software, so you don't lose any of your pixels to the crop. Though your final image would have greater vertical than horizontal DPI.
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