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.