I've heard that before, my only guess is that it is a machine, and like a car, turning it on and off is "bad" for it. Although it is a way of life, I couldn't see any way that you would actually find this out though. You could probably do a test to find out, but I would think it'd take 10 years of turning a computer off and on 500 times a day to actually take effect, but by then your computer would most likely be dead heh.
I leave my computer on almost 24/7 with my ftp's open and so fourth, bit torrent's etc... So people can download (gotta share what you got from someone else)