Mains cabling is not designed for high speed data at frequencies of many MHz - it's designed for mains power at a frequency of 50Hz. If you send high frequencies through it, they radiate for miles in every direction using your house wiring as one huge aerial.
This radiation is at a vastly higher level than that produced by wireless routers, and can cause havoc with amateur radio amongst other things. I've posted about this before:
viewtopic.php?f=26&t=4463If anyone complains about the interference, you'll have to switch them off. They're illegal to operate if they're causing a nuisance.
Cat5 on the other hand is capable of 1gig data transfers (which HomePlug would probably advertise as up-to 5gig) with absolute consistency and minimal EMI pollution.
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