The fact remains that when you restart your system (or at least turn it off every 4-8 hours) you refresh the system's valuable memory (ram) since RAM cannot hold it's memory without a constant stream of electricity, every time you shut down or restart, you refresh the RAM.
There are other factors to consider like fan life, HDD activity, not to mention constant strain on power supply.
I never said that you can't leave it on, I just prefer my PC to be fresh!
So, in the long run, I'd rather take the time to shut down every so often.
That is unless you run your own dedicated server....