I think I've posted on something like this before too.
The core issue is that you can't link zones for video playback. I get why (syncing the video playback would be very difficult, and how would you determine where to send the video or which "zone" wins, when both have a display). It doesn't really have anything to do with SPDIF and HDMI outputs in particular. There is no way to link zones when you are playing video.
However, it would be convenient to have a solution. I can think of a few circumstances when this would be handy. For example:
I have a new receiver that has multiple zone support. I'd like to be able to hook up some speakers in the kitchen, so that when I'm cooking, I can listen to stuff. This works fine if I want to listen to something other than what is being watched in the living room. It also works fine for audio-only playback. But, if I want to watch a show in the Living Room, and have the audio from that show routed to the kitchen too, then there isn't a simple or elegant way to accomplish this from inside MC*. This is probably the most common multi-room setup needed out there, honestly. You can see the TV from the Kitchen, and don't really care much anyway, but just don't want to miss the dialog (or keep listening to the news) when you get up to get a new bourbon and ginger (or, you know, cooking). My parent's house has this kind of setup, but MC couldn't easily run this kind of thing.
In these circumstances, the video itself doesn't matter. You just want the audio to go to the "secondary zone". Perhaps the zone system could be modified to simply allow this (drop the video on any "linked zone"). It would be super-cool if you could also link video zones on separate machines (through MCWS), so that you could have a setup where you play a video all over the whole house, and it is playing in any room where you walk in (assuming those zones are driven by separate PCs and copies of MC), but that seems like an extreme edge-case, and no reason to "hold back" audio-only video playback on secondary zones. MC knows how to handle the audio routing and sync already, so the "kitchen zone" setup is the low-hanging-fruit.
* Since my Living Room setup is a 5.1 setup and my amp is a 7.1 amp, I could use the extra two channels on the HDMI to pull it off, but this requires some fancy routing in both MC and on my receiver. You could do the same thing by splitting the Front L/R channel input if you use analog outs from the PC, but this would also be clunky.