Understood, but I think at least MythTV has the potential to be well integrated (not familiar enough with VDR to comment). Mythtv completely seperates the backend from the frontend. There are various other 'frontends' that talk to the backend. I believe linuxMCE may need to entirely replace the frontEnd but there is no theoretical problem with that as far as I can see. At the moment we are relying on functionality that is only found in the frontend but perhaps that can change.
Sure vdr can be run as a backend too... but the fact is you cant build a suitable UI (at least I dont believe you can) by then just building a few new Orbiter screens etc and 'plumbing' them in. Our feeling increasingly is that the Orbiter needs significantly extending to allow for this as...and thats a non-trivial endeavour ;-)