Didn't Mythbusters do the turning on/off the light vs. lifetime myth? And busted it?
Now, the argument for CFL's may be valid, I believe it says on the packaging they last longest if left on for long periods.
Fluorescents will definitely suffer from repeated on/off cycles. Looking at data from Philips on their lights (30k hours at 12hrs per turn-on, and 10k hours at 4hrs per turn-on), and my own testing with a CFL on a relay (something I had to do for a project at work, proving something to someone), each turn-on will remove 30-60 minutes of life from a fluorescent tube. This will be a bit different for different startup types. Instant-start is the harshest, because it needs to apply a higher voltage to the electrodes to strike an arc quickly; each time damages the electrodes a tiny bit. Slower startup methods, or else the newer programmed-start ballasts, can give longer tube lifespans.
Incandescents may also experience some depleted life from the thermal cycling of repeated on/off cycles, but I don't have the data here to say how bad it is. There was something done at work where we needed to slowly flash large numbers of halogen incandescent lamps. It was found that keeping them slightly powered during the "off" portion of the flashing would help increase the bulb life, versus allowing them to completely turn off between flashes.
But we're talking maybe 30-60 power cycles every minute, against a 750-1000hr total lifespan. A household light isn't going see that kind of cycle:on-time ratio.
The reasons stated by C1 I think are valid, in addition, the obvious benefit of energy costs. My machine is overclocked so having it off really saves on power, it also has an SSD with the BIOS optimized for boot up speed.
My HTPC is set up to sleep unless its downloading, and I have WOL setup if I need SSH access.
One benefit is missing though, security. If you computer is off, it's not being remotely controlled...
I'd say to turn it off if you're heading off to bed. If you really want it ready for you in the morning, set the RTC alarm in the BIOS for it to turn on at a set time each morning. If the PC's not doing anything useful at night, it's just sitting there feeding money to the electric company. If you've got a distributed computing project going, that's something else, though you should still be mindful that that's going to pull a good bit more power to keep the processor busy, though it will at least be doing something productive.
While there will be some stresses developed from thermal cycling, I would also not expect them to be substantial. The big problems you start to see are things in outdoor environments, where the electronics can drop well below freezing, and then back up above 140°F.