I think the problem with this policy is that if it started getting enforced with any vigour at BA (and so far it doesn't seem to be), people will just resort to bringing external battery power blocks to charge their devices from overnight instead.
I'd be more worried about that personally, given where those devices come from (I've had at least 3 of them as freebies lately, each more dodgy than the last), and it's something the crew can't do much about until it's too late if the device is squirrels away in a seat back pocket or the CW drawer. I think the in seat power (which is hopefully tested regularly!) is the lesser of two evils there.