It is kind of weird, because I think the biggest problem with leaving the computer on all the time is the worry of power surge that can kill a computer or destroy components like power supplies or worse, logic boards.
Other than that, I leave my computer on all the time because in OS 10, it takes forever to start up. On my Classic machines, I turn them off because they start up in 43 seconds flat. LOL.
I have installed power conditioners and surge protectors in my studio so I am not worried about power surge. But I have seen dead computers before when I was working for Apple Centre.