Re: Turning them on and off shortens the life?

Before you ever notice it has shortened their lifes, you will have replaced them ten years ago. And the amount of power to boot them up is nothing compared to what the typical machine consumes when left running the whole night.

And re: physics. Your reasoning is, well, peculiar. The by-product heat generated by a running PC is a magnitude more expensive than that of a building's heating system.