I also don't see much reason for a 6x12 back, but a 6x17 back (I just bought one for around $500) that fits a 4x5" camera is a different thing, because 6x17 is wider than 5". If you price out the cost of a 617 camera with interchangeable lenses and camera movements, then the DaYi 617 back starts looking like a real bargain, particularly if you've already got a camera with a Graflok type back and a few lenses to cover the format.
For me, the 617 back extends the range of what I can do with my existing 4x5" equipment, so it's a good value. I decided to upgrade my 75mm lens to one with more coverage for 6x17, but that was probably going to happen eventually anyway.
I'm not sure what the attraction of the new KangTai back would be over the less costly DaYi. They look fairly similar, but sometimes these differences aren't always obvious.