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 I used to build all my desktop computers, I never turned them off, reason is the Airplane Syndrome- If it's going to crash, it will happen on take off or landing.    The more often you powerup/powerdown the more chance there is of a crash.  When the computer goes to sleep, it doesn't use no more energy than a 7 watt light bulb.  And the convenience of not having to go through a boot sequence.  also, another tidbit of information.  if you picked up a virus, like other software, before it can take full effect it the computer must be rebooted.  Obviously, viruses do not 'advise' you that your cumputer needs to be rebooted before it can wreck your life.  However, at some point, you will install a new app and it will require a reboot. 

I don't use desktops anymore, I just drag my laptop around, but even my laptop I rarely shut down.