Syncing simultaneous output devices is not a trivial task. Network latency can introduce just enough delays to make it noticeable when in the range of two overlapping zones. Users *regularly* complain about this for devices like Tivo and other DVRs when watching TV from one versus having broadcast TV or a non-DVR tuner. The delay is minimal but it's enough that everyone complains. There's nothing that can be done to solve it, at least not in the current quantum physics rules the universe operates within. That and customer complaints or inquiries cost money; both in support costs and negative PR.
Then there's also the issue of pumping out simultaneously to different devices. If they're all listening to the same source in some sort of shared 'stream' mode then each one's stuck without being able to stop/start the playback. Again, customers complain even though this isn't the 'fault' of the software or devices.
I suppose it might be possible to push the audio output to a streamed channel via multicast but I'm not sure how many devices or clients exist that would support this easily, let alone reliably.
In short, it's cheaper and easier to just run speaker wire. Cheaper in parts as in-room speakers and volume controls are a helluva lot less expensive than PCs or media extender style devices. Cheaper in support costs too.
We're in the process of installing a CAV6.6 and A-bus system. Control will be either from the keypad and IR remotes into Girder or from a trio of touchpads running NetRemote, strategically placed around the house as well as the console running MC11 itself. MC11 will push 4 of the zones into the CAV (out of 6) from USB audio devices and the other two come from a DirecTivo and the home theatre stack of devices (tapes, vcrs, dvd, cd changer).
Wall keypads for volume control are no-brainers to operate, for us or our guests. Doing track selection via a wall panel pretty much sucks no matter whose small form-factor panel you try to use. That and putting in the sort of 1024x768 touchscreens needed for MC11 is hideously expensive. At that point one might as well just go with a Crestron system.