The power button on the back makes the iMac neater from the front. If the shortcuts are used, we do not need to use the power button that often. If the iMac is put into sleep when not being in use. The actual use of the power button is even less.
A Cinema/Thunderbolt Display even does not have a power button!
So here is a silly question, I've owned a G4 and now G5 tower, both have a 'throbbing' light on the front when it's sleeping, does the current iMac have anything like that to tell you its asleep and not off?