I'm thinking about turning sources on and off as I move around.
Right now, I can send local content to a specific device in a specific room via Plex (or XBMC, depending). Which is cool all by itself.
But the cooler thing would be a relatively seamless handoff between receivers so that for example I don't have two different media players going at the same time, or one turned up to unfriendly volumes so I can hear it from another room.
802.11 does indeed cover my apartment, but there's still the matter of location awareness, and also of having the most convenient control, which is usually going to be a hand-held device.
This isn't that much of a hassle in my little apartment, but I could see it being very useful for a family with a good sized house and a couple cars with reasonably modern head units.