It's nice for people to check email, web surf, YouTube, Facebook, etc...but people use even lower powered devices (and much more convenient) such as iPad for those things now days. Anything else that requires a decent amount of computing power, you probably save more power and time by having powerful hardware such as i5 or i7 and getting the job done quickly therefore more efficient. Idle power draw isn't that big even with an i7. I see very small group of people who would purchase this product, really.
You could buy a Mac mini base model and hook up an IPS display, which would total about the same as ULV iMac. But Mac mini gives you expandability, more bang for the buck and be just as power efficient.