Just a point here Ian - properly implemented video circuitry does not degrade the picture in any visible way, nor need it interfere with the sound. I am talking about products in the AV8 class though - your average receiver is another kettle of fish. And you do get the advantage of a decent OSD that can be superposed on the main picture (without any loss of quality if it uses a broadcast quality pixel switch like the AV8) so you can see what the processor is doing at any time. As processors get more complex this is becoming essential IMO.
Finally there is no way you can sell decent numbers of processors without video IMO. I believe the reason other manufacturers leave them out is either for reasons of back panel space and/or cost or because they don't know how to build them properly (good video design is very different from audio design). Except for reasons of space there is no reason to put them in another box - in fact it will usually add quite a lot to the overall system cost.