I consider other components than the hard drive when answering this question. LCDs, as Ken mentioned, have a limited life, they also dim over time.
So you want the LCD to sleep even when the computer's running.
But Fans, RAM, and even CPUs have a life cycle that eventually ends.
Hard drives are so inexpensive, and are the second-most-often upgraded component (next to RAM), that I don't consider the failure of a hard drive 6 months or even a year before I expect it a big deal. I have a 40 GB hard drive in my notebook, that runs at 5400 RPM. I'll want an 80GB, 7200 RPM in 12-18 months anyway.
So I don't have too much pity on the hard drive. I hibernate regularly since this lets the CPU, fans, and RAM rest as well.