I don't understand why everyone thinks they're worthless because they may ruin a screen. Don't close the laptop with the cover in place and take it out first. I realize this isn't convenient, but they are still very useful if you happen to spill something. I bet anyone who has water damaged their notebook through the keyboard before wishes they had a cheap keyboard cover in place at the time.
Besides, they're extremely inexpensive. They are even included in the purchase of most hard shell cases simply as a bonus.