i leave my macbook on 24/7, it does not put anymore wear or tear on the harddrives or anything for that matter, then turning on/off or waking it from sleep does. Computers are meant to be stayed on, and have no efect if u leave them on 24/7 it wont shorten the life on the computer.
That's fine if you want, but I don't believe it is correct.
If the hard drives are being spun down while the computer is running (that is, the box is checked in Energy Saver) then leaving it on is no different from sleep--either way, the drive isn't spinning, and you're just wasting electricity on everything else. Powered off entirely is the same, except then the drive's circuitry isn't "warmed up" either.
Hard drives and fans are physical things that spin. If it involves physical motion, eventually it WILL break. May be in a day, may be in 50 years, but it WILL break some day.
The only real question when it comes to longevity is whether the thermal cycling of off-on and the physical wear involved in having the moving part stop and start again are "worse" than leaving it in a constantly moving and roughly temperature-stable state.
A car's engine, for example, probably would not like being stopped and started every 30 seconds, because of oil flow and such. But obviously it extends the life of the car to turn it off when you're not driving it--I'd be willing to bet money that if you let your car idle all day instead of turning it off, it wouldn't last anywhere NEAR as long.
Hard drives, based on the fact that they now sleep and wake constantly, are obviously designed to take a lot of power cycles without issue. I'll also point to the fact that recently several drive manufacturers switched the warranty on their consumer drives to assume 8 hours of use per day, rather than 24.
Whether this was just CYA or something real, I don't know, but I'd take it to imply that power-on hours are, after some point, worse than a power cycle.
For fans, definitely so--the bearings wear faster if the fan is moving faster so far as I know, although the general lifespan is long.
The bottom line is you are probably not doing yourself any favors by leaving a computer--especially a laptop--on all the time. Add to that that "heat is the enemy", and the tight quarters of a laptop, and you're probably baking the drive faster than it would be if you just turned the thing off or slept it when you weren't using it.
On an unrelated note, if you use your computer 12 hours a day, turning off a laptop when not in use would save US$15-20 per year depending on how much you pay for electricity. A desktop could easily hit $100 per year, more if you do folding or something on it.
Basically, sleeping my G5 tower when not in use nets me enough to buy a new hard drive once a year.
