I think all power lines should have a communications protocol. Not every device has to use them, and they'd be lower bandwidth than data-only lines, but I think the era of data-communicating appliances is here to stay, and it'd be better they go over the wires we have rather than clogging up the wireless spectrum.
Within a single building for sure, but even neighborhood lines could carry some data I bet. I wonder if and how that would work. Gonna have to learn some more about electricity and grid infrastructure.