Unless you're going to also want to be able to use a film back, is there any advantage of the ZD back to the ZD one piece camera? I guess ability to upgrade if/when you want to use a different digital back. But there's a substantial weight difference between the two. The ZD one piece camera weighs about 3 lbs, I think, and the 645 D with ZD back about 5 lbs.
With a back you have one more piece which can be off. The one piece camera just needs the lenses and the screen calibrated to offer most accurate focus.
The back can be used on more than one camera, a Sinar or Arca for instance