I turn my PC off anytime I know I wont be using it again within 1 hour, and in 20+ years of computing, I've only ever had 1 PSU fail during power up, and no other serious faults to report. A few bad sectors on hard drives, and a few faults caused by overheating and failed fans etc, but nothing to be concerned about.
If an average PC uses 150W of power (Most people totally overestimate their PC's power requirements.) then it will use 1KWh (1 unit) of power every 6.6 hours. Thats 3.6 units per day, or around 1300 units per year per computer.
True thats not a 'huge' power requirement, but it still adds up. Around £120 a year depending on your unit cost.
Those costs will increase if the computer isnt idle ie folding/boink etc, although downloading uses very little CPU power, so a download rig is pretty much minimum power.
As I have 3 computers at home, its worth my while to switch them off when not in use. And I dont believe Im just being 'cheap' I leave all my Hi'Fi amps powered up 24/7 even though I know that its using 140W of power just to 'heat' my home. Thing is, the hifi doesnt quite sound right until its warmed up for an hour, whereas the computer is ready to go 1 minute after I turn it on.