Yeah, but spinning them all the time will wear them down faster, though admittedly, by that time, one is usually into a new computer already...
... in the times I need my home computer at intermittent times, I leave it on and have HD power down kick in after like two hours of nonuse or so. Usually, though, I just shut the system down until I need it again, though the drawback of that is the restarting of the HDs and increased powerspiking to the PSU, shortening the service life of both.
But I have to watch my power costs these days very carefully.