this reminds me, when I was a kid, my dad would freak out if we left the Atari 2600 adapter plugged in all the time. We would always have to unplug it after every use because he heard somewhere that the adapters burn out over time when left plugged in. I think this is where this stems from. I think there is some truth to it, even though I've never had an adapter burn out on me from being left plugged in the wall when not in use, those old adapters weren't very reliable.
Even if it was true back then, that was 25 years ago, I'd like to think technology has advanced to the point that your home electronics can be left plugged into an oultet when not in use with no danger of them breaking. Even so, I tempt fate to this day by leaving my old NES and Atari 2600 adapters plugged in at all times, if for no other reason then to spite my dad 25 years later.