I would like to commend Sinar and, of course, Rainer, for joining together to develop such an innovative product. It reflects a high degree of collaboration with an exceptionally skilled working photographer who understands exactly what he needs. If it turns out it can only be used with Sinar and Leaf AFI backs, I understand the business imperative behind that decision(much as I would like to buy one to use with a Hasselblad back). Sinar provided all of the R&D funding for the camera's development. Sinar should be allowed to make the business judgment as to how to best maximize its return from the R&D investment. It is an uphill struggle for Hasselblad, Leaf and Sinar to get prospective buyers to take a close look at their products and test them out. By offering a camera or lenses that you can only use if you buy one of their digital backs, they at least put themselves in a position where someone will take a much closer look at their backs and not reflexively opt for a Phase back.
If anyone who owns a Phase back is pissed, I would suggest flooding Phase One with e-mails complaining about its lack of product innovation(unless you think the LCD on the Plus Series backs or the "update" of the AFDII qualifies as product innovation). Phase One has been the 800 pound gorilla in this market. Why did Phase not work with Arca or Alpa and fund the development of such a camera? The answer, by the way, is that Phase has enjoyed a nice long run of others doing the dirty work of camera and lens development and marketing with its low profit margin(on the road to bankruptcy) while Phase sat back and scooped the profits from its near monopoly position of selling high profit margin digital backs. What a great business model. For Phase! Code word for that ruse: "open architecture".
This latest development tells me, at least, that MR's advice in his Phase camera review that a prospective buyer of medium format digital should first decide on the digital back is incorrect. I believe you should first decide on what your photographic needs are. There may be some cameras, lenses or features that you can only use if you purchase a specific back. Given how close in performance the various digital backs are now, why would I pick Back X if I really wanted to use Camera Y? Of course, the same goes for certain features of digital backs. If you need long exposure capability, a Phase back is apparently the only game in town.