The issue, I think, is that you're extrapolating from the usecase where high bandwidth ethernet is carried over the power lines. Protocols' like X10 bandwidth use and needs are so low (dozens of bits per second) that the interference is indistinguishable from the regular power fluctuations.
I speak from experience. I tried powerline ethernet because wifi signals traverse the house walls very poorly, to say the least. The sheer noise the powerline adapters generated into the line, regardless traffic, was unacceptable.
