I'm voting with the "leave it on" brigade. My creaking old 233Mhz Pentium has lasted 5 years with as few system deadstarts as possible, usually only required when WIN95b gets so broken I can't do a soft-start.
HDDs usually snuff it at spin-up time after deadstart...motors burn out. Fewer starts mean the drives might actually go to TBO.
Monitors usually die when the flyback transformer that generates the EHT for the screen fails, due to overheating. Either goes open-circuit, or the soldered joints on the main board go dry. I have a fan on a small cardboard chimney on the top of the monitor casing to keep the internals several degrees cooler. Seems to work well.
Avoiding the "upgrade treadmill" is one goal, I try to get 5 years from a system before a major upgrade/rebuild.