I run one because I have a supercharger---so if I toss a belt for some reason, I can still drive the car to get someplace safe. With the stock water pump, if you throw a belt your going to overheat in minutes. I know one guy that was able to drive 20 minutes and make it home only because he had the electric water pump---would have never made it without it. Obviously if your n/a, you'll never have to worry about tossing a belt, but with superchargers it is a possibility and I even carry an extra belt in the trunk.
I have heard of some people carrying an extra relay for the water pump, but I haven't and mine is a daily driver--I have 71K miles, and have put about 12k mile on the electric water pump so far.