One of the V issues was lack of eltric connections. I believe the 555 body worked quite well with the Kodak back (some years back) so I assume that the need for cables is simply a lack of communication shutter to back.
It is one of the reasons that I switched to Contax for my Hasselblad lenses.
That said, the V later bodies should only need the shutter trip cable to communicate with the Phase back. The phase back DOES need a wake up though, so I wonder if there is a second fire needed.