Funny you should bring up this subject, I used to leave my PC on for days or switch it off but leave it plugged in and on at the mains that was until about a month ago, I had been having a problem I couldn't fix, my PC would boot then reset itself then hang on the boot screen or give silly error messages, after reformatting SEVERAL TIMES, changing hardware, cleaning my PC etc I could never solve it. it would run alright for a few days then start playing up again, then I decided to try something different, turning off my PC at the tower and switching off at the mains as well until my next usage. Guess what? yeah you guessed it problem solved, no more PC problems, so I have come to the conclusion that switching off your PC is essential to keeping it running healthy as well as making your components last longer.
From all that you can see I now turn off my PC everyday and am allot happier for it.