The two schools of thought I have heard go like this.
1) Leaving it on all the time wears out the bearings in the fans and hard drive (if you don't power it down). This constant high speed revolution eventually causes them to break.
2) If you turn it off and on all the time then there is temperature changes going on inside the case. It gets hot when you have it on and then cools only to repeat the cycle over and over. This constant change causes the hardware to expand and contract which eventually breaks solder and the circuits lose thier integrity.
I would think these ideas would apply to a laptop the same as they would to a regular PC. So if you enable power saving devices then, in theory, the only thing that is damaged is the fans which are cheap.
Personally, I don't like power saving features as it seems computers never like to wake up from them, forcing me to reboot. Remember, rebooting shouldn't be that stressful of a task. I think most computers like to reboot due to the clearing of physical memory, enabling more system resources to be available.
All in all, though, in my opinion, it doesn't really matter. The hardware is going to be obsolete due to faster stuff coming out way before the bearings wear out or the boards begin to crack due to thermal expansion.
Hope all this helped. Justin
Feel free to email me at:
beckham@mailbox.orst.edu