This is what was wrong with it. We couldn't figure it out in the store, now I know!
Nick, I think you may be dismissing the Flexbody far too fast.
I have two of them, including one that has been modified so it also has sideways shift and tilt (a "BendyBlad"). I have used them with Aptus 17, 22 and 75 backs and, now, with a Credo 60. I have no problems attaching backs and I suggest you test "your" one to see if it will accept digital backs before you dismiss it.
I have never heard of the problem that Yair mentions. Digital backs are designed to clip onto Hasseblad bodies just like film backs and I cannot understand why there should be a problem with a digital back unless the back's shape protrudes where a film back does not, which would be very strange indeed. However, I accept there have been problems with some Flexbodys and you need to research further.
There are problems with the HTS device, in my opinion. First, it is horribly expensive and, secondly, it has a lens factor of 1.5 (you need to check this figure). So, although you want to use the HTS with wide angle lenses, you are compromised because a 40mm lens becomes a 60mm lens, and so on.
The Flexbody also has up to (from memory) 21mm continuous extension capability built in. You need to check whether the HTS has this capability (I think it does not).
I find using a Flexbody with the Credo is great. I do not have to use the optical viewfinder and I leave the back on the Flexbody all the time. I use LiveView to compose and focus and it is a fantastic to be able to zoom in to 100% view when you want to focus.
Because of LiveView, using the HTS with an H series body does not give you anything extra apart from the aperture in EXIF. The Credo records an approx shutter speed which I find close enough. With both the HTS and the Flexbody, you have to manually record the tilt and shift. [I have since found this is incorrect and the HTS records tilt and shift data--see Nick-T's post below]
I enjoy using the Flexbody so much, I no longer take a "normal" camera body into the field. I appreciate the process of using the Flexbody and the extra movements it provides.
So, I suggest you consider the Flexbody a lot more. They are great devices and can serve as a backup for your normal camera body if pushed.