My parents never used them - they were of the opinion the covers drew attention to the sockets.
[NB for this this tale - my parents are educated, were caring parents and didn't take any risks with us. The below happened in about 3 minutes of me in the living room and mum in the kitchen.]
A famous family tale - when I was about 5 I unhooked my Dad's keys from the hooks in the hall with an umbrella, and decided to play canal boats. I remember this vividly. I was using the dark triangular space behind the sofa (the sofa had an angled back) as the interior of the boat.
I knew that boats had engines started by keys. You can see where I'm heading with this right?
I stuck the keys in a socket, pretending they were an ignition. Very unfortunately for me, they were a large bunch and I was able to stick one in the top, and one in the live. I went BANG and all the lights went out! Flew across the room, destroyed the socket, the keys and all the electrics on that circuit.
This was the early 90's and fortunately dad had a brand new swish Ford Escort, and it was the first ever car they'd owned with a thick plastic key fob - which thank god, I was using in the live slot, so I didn't receive an electric shock directly. It would have undoubtedly killed me.
I'm of the opinion that sockets themselves are fine - just keep the objects you can stick in them well away from small hands.