Firstly, I always turn my electronics off after using them; primarily to convince myself that I'm being good and saving energy/the planet.
However, the argument for leaving PCs on is very persuasive (assuming that you turn the screen off and leave the processor on standby):
Much like a car, the average computer being used by the average user is under the most (physical) strain when starting up.
Standby mode uses almost no power, and puts much less physical strain on the CPU when restarting.
Expensive things like graphics and sound cards are all but unaffected either way.
Power surges from turning on/off the mains power *used* to have the potential to damage the RAM or even the BIOS (iirc). Nowadays this is not a problem, but it still remains in some peoples' minds.
"Getting hot" should not be a factor on systems under 5 years old; if your PC's fan isn't able to keep up with the heat produced by your processor at maximum capacity then you might have problems regardless of how well you treat your computer.
IMO, The major problem is actually software. Some applications can really hemorrhage memory if you install/update them and use them without restarting. This can have a significant effect on performance (check iTunes RAM footprint after an update but before restarting - on some systems it's about 30mb bigger). Force-quitting seems unnecessarily cumbersome (and occasionally ineffective) in that regard.
Consoles and Laptops (particularly the 360 and Gamecube) aren't built to handle 24/7 operation, even if the strain is minimal. With that said, I've read forum posts claiming that leaving a 360 on 24/7 actually reduces the likelihood of a RRoD - and, of course, others that claim the opposite.
TL;DR - no conclusive evidence either way. Waste of a post. Sorry.
