Great post and good points. Amazing how different the experience can be depending on the environment and point of view. I worked in a hospital environment for 6 years supporting over 500 desktop systems and consistently found those users who had shut their pc's down nightly had fewer hardware failures than those who left them running 24/7. My conclusion boiled down to moving parts, dirt, and heat. The most commom failure for pc's running all the time were hard drive failures, processor fan failures, and power supply fan failures. My theory on why people complain about a pc that "worked fine yesterday but won't work when I turn it on today" (that happened here too) is that a hard drive failure occurs from years of overuse and develops(failed bearings or motor, read/write heads, bad clusters from crashing read/write heads)while the computer is running. Well, the OS and apps are already loaded into RAM. On the next reboot, the drive can't read the data and errors occur. And never rebooting isn't going to work either as everyone knows. The hard drive failure rate appears to be lower for a pc running 50 hours a week than one running 168 hours a week. (yes, one can set the pc to turn the hard drive off after a period of inactivity which worked sporadically in the windows 98 days, works great on windows xp - but is negated by processes like disk indexing and robust antivirus software that turn the disk back on within seconds of it being turned off.) I agree that constant cycling of hardware on and off is stressful to the parts. But one on / off cycle a day seems to be less harmful than the effect of running constantly.
The other issue is dirt. Contrary to popular belief, hospitals are not ultra-clean environments (except in the operating rooms). Computers running constantly build up a blanket of dust on the heatsink and intake grills very quickly. Thus, overheating, then component failures. This buildup happens a lot slower when the pc isn't running.
The energy it takes to get something going IS greater than the energy it takes to keep it going - but only for a few seconds or minutes at most. I would hesitate to leave my car running all day to avoid the stress of restarting it when I left the office.
We don't have a policy here about shutting down. We use E policy orchestrator background service to update antivirus dats and Novell push agents for other updates, so pc's will get updates while they are running or the next time they are turned on. If people like to keep their pc on, we just ask them to turn it off for the weekend at least.
Good luck