I turn all my pc's off at night, and I don't let them run unless I'm using them, or I'm going to be using them.
Moving parts, motors, bearings, etc. will only turn so many times before they fail. Not to mention, the pc sitting there inhaling dust and getting filled with crud.
Do you leave your car running if you're not using it?
if my car didn't take 2.98 a gallon and if there were no moving parts, sure, why not