*cough* S3 standby *cough*
That still uses power (and I don't remember my wintel machine have that possibility). Also, a lot of things like PSUs (like those for f.i. cellphones and computerspeakers) still use power when they are not in use, which I find a complete waste.
All things in my hobbyroom (computers, printers, monitors, consoles, tv, guitaramps) are connected to three main switches . When nothing is in use, all power goes off to save on electricity.
Besides, what's the point in leaving a computer on when you're not using it for several hours (not using meaning not using - so no downloading/running a server or anything)?
And no, I'm some environmental tree-hugging hippie. I just don't like wasting money. :-)