I think the only time my PC doesn't have power is when I need to move it, or am doing doing maintenance e.g. replacing a HD, or cleaning out the CPU heatsink (though I sometimes do the latter with the machine on

). Otherwise, it's on 24/7 for the entire year.
Yes, the components will wear out faster (more likely mechinal parts), since you're using them more, and everything has a limited lifetime. I've already had to replace the oldest HD, and the power supply unit itself, over the course of 3 years I've had the PC (both within the last 2 months). Considering the latter was rated at only 20,000 hour working lifetime though, that's actually pretty good

I've not had to replace my CPU fan, but the fan on my graphics card has been broken for maybe 2 years now - I haven't bothered replace it and instead underclocked the card so it generates less heat, since I rarely use 3D-intensive things like games
Whether it wastes electricity or not depends on your setup and how you use your PC really. If you need it on 24/7, or have it doing something at all times, then I'd say it's less of a waste than someone that doesn't need to run a PC like that all the time, like someone that only uses a PC for email/web surfing, and just leave it on for the sake of it. Of course it'll use more electricity overall, but you have power saving options to help counter that to some extent, like turning off the HD/monitor if the PC is idle for some time. I physically switch the monitor off usually when I'm not at the PC anyway.
Hibernation is the same as turning the PC off completely powerwise - when the PC is in hibernation, the RAM is saved to HD, the PC turns off, and you use the power switch to turn it back on again. The difference from a normal shutdown is that instead of booting normally, it'll reload the contents of RAM instead and so you can resume exactly as you left it before hibernating. It's not the same as leaving the PC on 24/7 - the PC can't do anything while hibernating. If you use hibernation, you're effectively messing around switching the PC off and on all the time
The plugs will only overload if you're stupid and do something like plug in PC + monitor + speakers + printer + scanner + toaster + stereo system + other devices into a single socket

Use several sockets instead. If you're worried about power surges from the electricity grid itself, get a power strip that has surge protection. If you're worried about short brown-outs/black-outs/power-cuts, then maybe a Uninterruptable Power Supply. My UPS has surge protection as well, so I only need the latter.
There's probably lots of other factors to take into account too, but I have no problem leaving my PC on all the time without a side cover
