There was another thread on this very subject, which you might want check out:http://www.geekstogo...s...=103710&hl=
There are reasons to leave it on and reasons to turn it off and ultimately it is a decision which you need to make based on which works best for you.
It is true that there is harm done when the machine is power cycled, but if you leave the computer on all the time and don't have good power conditioning and good temperature control, you can have problems due to that and since it is on all the time, there is more of a chance of having such a problem. Also, some parts, like a disk drive, will wear if left spinning, as well as fans. If a fan goes out and you are not around to notice, you could come back to toast. There is also the issue of possible, although rare, extreme failure which could result in fire. That means if you leave it on all the time you should also have a fire system and/or ensure that the machine is on non-burnable materials and nothing around it could catch fire. While it might be rare, it is something to consider.
I leave my computer on if I am going to use it again in a couple of hours, otherwise I turn it off. I am careful to have it off if the room temperature gets too high and I have it on a UPS. With a real computer room and someone else paying the electric bills, I would most likely leave it on all the time.