There are many other condiserations, which I'm very surprised that no one has yet mentioned. Here are four that come immediately to mind:
(1) Moving parts such as fans and spinning hard drives wear out. I'ts a shame to come home to a dead computer because the CPU fan took a dump during our absence, or a hard drive spindle wobbled or ceased during an idle head movement.
(2) Power hits or fluctuations from thunderstorms and overloaded power grids can kill computer electronics dead. Surge supressors won't completely protect you, and how many of us have our systems on a UPS? How many have both? When is the last time we checked the fault light?
(3) Dust accumulates everywhere and clogs CPU and chipset heat sinks and fans, reducing their efficiency. How many of us pull the covers periodically to perform PM's (preventive maintenance) on our systems, or our less knowlegable friends and family?
(4) Power consumed needlessly is a huge problem in this age of global warming. Here in the USA, we represent 5% of mother earth's population, yet we consume 25% of the energy. We won't even bother to inconvenience ourselves by turning off the light when we leave the room. What example does this set for others? For our kids? Something's drastically wrong with this picture. Things have to change.
Leave my computer on? No. Absolutely not.