We have a plasma, 2 1/2 years old and very nice it is to, but they will be in the minority it's true.
The power consumption does vary - presumably in response to the image.
But the whole, they could tell what TV program you are watching, or whatever seems to be a rather silly argument. OK, maybe it's something that theoretically be done - but it isn't, and couldn't right now with the way they work. And what is the purpose for this? If there is a real concern about them being used in the future for some sort of spying on people then the response to that is to limit what they can do.
Anyway - what about DVD's , streaming, recorded programs, etc. etc.