They're designed to be switched on and off. It does indeed shorten their life, but only to their design life - not to an unreasonably short time, and they're better than they were. It's primarily down to the caps in the powersupply AFAIK, rapid deep charging and discharging of the electrolytic caps isn't helpful for them, they prefer to remain charged but dont like warm temps either. They're rated at something like 15 years at 40 odd degrees. But as I say, they're designed to be turned on and off daily, so it should not increase deaths unless your company holds onto PCs much longer than they should!
The major source of failure on computers is PSUs and hard drives, hard drives dont like being hot - thats a case design problem and overnight they'll be cooler than during the working day, and again they're not keen on being started and stopped but its not really a factor in their life. Google completed a test on millions of their drive failures and found most of them died, IIRC, because of heat problems, not related to power-ons.
Ultimately the answer is turn them off if possible, hibernate if you need a fast boot.
Leaving a computer on doesn't slow apps down these days, my system was running for 2 months (XP pro) until yesterday without powerdown (its also a license server) and after reboot it was no different - thats a problem from the old windows 98 days.
Decent software is available to do the trick. Free software is available to. But it'd be better from a network point of view.