The arguement for leaving a PC powered on (and/or sleeping) is that going from a cold board to powered up causes the wear and tear. It's seen as a question of the number cold to hot cycles and then back to cold is a strain on the circuitry compared to a more constant temp. Which may be true but chances are something will go at sometime anyhow.
I'm on 24/7 and willing to accept the consequences of my reckless actions.
Server:ASUS F1A75-V Pro,Radeon HD 6000,AMD A6 3670K,SSD128GB,2xWD 2Tb,W7Ultx64,5.1spks,2xHaup2250-MC,ATI650,CM7777,WG HD-8800
Client:P4 3.4,ATI HD4650,W7Ult
nDroid Client:Samsung Galaxy Note(synced to previous days recordings)