My desktop and my server are on 24/7. The only time they're off is to restart for a software update, or a blackout. I leave my desktop on all the time because I hop on not long after I get up, and I don't want to wait for it to start up. A lot of the time, I'm downloading something that'll take a few days too, so turning the computer off isn't really an option. As for my server, well, it's a server. I let a bunch of people use it, and they may need to access it at any time, so it's left on all the time.
But I turn the hard drives off when the computers are inactive. Leaving your computer on all day, every day, will kill your hard drives faster if you don't. Technically, it's not any faster, since the hard drive is on for the same number of hours, but from the date it's installed to the date it dies, the drive won't last as long, since it's on for more hours of the day.
And I gotta say, losing terabytes of data is quite the PITA.