Indeed, there was quite a long thread about that in the offline discussions when we were looking at 2.0 .. I might still have that saved off somewhere, but it might take a while.
In short, we wanted to have some commonality between parts to ensure that there was some semantic that bound these parts together which would prevent a free for all mash-up. I seem to recall that the stereo pipe was good test case for things like display window, time code, aspect ratio etc ...
There was a desire to have a small enough set that could be potentially relaxes as time went on; going the other way is much more tricky.