My view on this is that everything has a certain amount of time that it's made to run. The more you run it, the more things wear out. I'm mostly concerned about fans and harddrives, but the video card and processor(espcially with overclocked systems) could also be a consideration.
Now, I used to leave my system in suspend all the time, but for some reason it won't stay in that mode anymore. But how my system would work, it would basically be completely off untill you pressed a button. Then it would be right back where you left it. No, not hibernation, suspend to ram as it's called in the bios.