In my family, powering off definitely has an advantage, and it shows. My father keeps his PC running 24/7, the only time it saw shut-eye is when there were rolling blackouts in CA. My sister does the same, 24/7 on bittorrent downloading anime, sucking up our bandwidth.
I'm the only one that shuts off my PC, but just recently I've started using stand-by. I like stand-by as it emulates a shut-down PC; there are no lights, no fans, absolute silence, but when I'm ready to go Windows is already loaded.
My father, bought his PC last year: P4 2.4GHz (533) Asus P4S533, X800Pro, and it just took a dump last week. Nearly lost all of his files and barely boots at all. My sister has gone through 3 PC's in one year. The one she's using now is my old P4 3.0E Prescott. PRESCOTT and she leaves the beast running 24/7, on the second floor of our home, in Southern CA! It'll probably be dead by February. I've had the Prescott for a year before I slid it off to my sister, and I've had this system for about 8 months. So.. we'll see how things turn out.