I can appreciate that people may be accustomed to turning off their computers. For maximum power savings, for the benefit of getting regular reboots, for the sake of parents who may have shouted, "Do you think we own stock in the electric company?"... not the worst thing to do.
Ironically, many laptop users believe they are powering off their systems when all they're actually doing is putting them to sleep (a brief press of the power button or shutting the lid puts them to sleep, it does not shut down).
I happen to be among the many who (intentionally) never turn computers off until a reboot is required (and modern OSes can run weeks or months without needing a reboot). I put mine to sleep. For security reasons, when they awake from sleep, they're set to require my login password. As Alrescha described earlier, mine are also set to wake for network access - the display stays off, but the CPU and hard disk will respond when other devices on the network need them. That's been part of the intent with Apple TV from the beginning - the computer functions as the Apple TV's server.
I worked in broadcasting for many years, so I'm very accustomed to having electronics that run 24/7, year in and year out. And then there are all my years in computing (both in broadcasting and afterwards) - most servers run 24/7 as well. There's a school of thought that says that electronic devices last longer when left constantly on, as there are stresses put on the components when the power is applied. For modern gear, though, I'm not sure whether the stresses are all that different, whether you leave things on constantly, or power-off regularly. At this point, even "Off" is not truly off - most systems stay on in a minimal way. For the sake of near-instant-on convenience, for the sake of being constantly aware in our constantly-connected computing environment... my computers stay on 24/7, and they last for a very long time.