One of our TV's is getting a bit old and doesn't have USB. Where it is mounted it is a bit of a pain to get another power lead to so for now I have just plugged it into the USB port on the back of the Virgin Tivo box.
It all works perfectly well - it remains powered all the time (even with tivo in standby mode) but I think I prefer it that way as that way the Chromecast can turn the TV on and change to the correct input.
Does anyone else use this USB port for anything? Can you envisage any issues?