My experience is the opposite for drives failing. I have seen more fail when cycled than when powered continuously . I have never had a single failure of a drive that was working, it always failed at the next power on.
hard drives are constantly spinning down and up anyway, they have built in sleep modes for when theyre idle. and what good is a harddrive that only works if its left spinning? its going to shut off at some point, and thats when youre going to lose your data. and a lot of times you can catch the problem before its too late (boot up in the morning, drive takes awhile to spin up and read. now you know its dying. but if its always on 24/7 its going to get so bad before it dies that once you turn it off you just instantly lost everything with no chance of recovery).
and furthermore, i think it was mentioned that drives are always accumulating dust and thats the real cause of most problems. the more idle time you give them the less dust they suck up.
and i also just want to note, THANK GOD for ssd's. thank my heavenly lord. now just cut the prices in half and were in business.