I USED to leave mine on for the "why spin the drive up every day" argument. However, due to my needing to be extremely frugal now, I started turning them off. And in fact, I am only using one. A side effect was that my electric bill went down $40 the first month of not running 5 computers 24/7.

But I used to leave them on mainly because of not wanting to spin that drive and move that drive head over the boot sector every day.