To all the people suggesting that you don't need plug covers, you've obviously never walked into a room to find your 8 month old slobbering over and licking a plugged in and live four way block. These may not be the best covers to protect against a shock, but they must be better than nothing. I'm currently weighing up these covers and others which seem to provide a better cover but are easier to pull out.
I'm currently renting and live in a house which doesn't have RCD's in the fuse box (a fuse protects cables and devices, an RCD protects people) so it's even more important I try and protect the baby.