Well, USB 2.0 capped at 500ma, while USB 3.0 raised that to 900ma. I still remember a plethora of devices that would plug into multiple USB ports just to draw enough power to operate... disk drives are the easiest example. There is a reason external drives don't power off the USB port, although doing so would not only make external drives significantly cheaper (no power brick), but the computer's power supply is much more efficient at power conversion than most cheap powerbricks anyway.
How many powerbricks are around on your desk? Cable modem, router, inkjet printer, laptop, one brick for each LCD monitor, a brick for each external hard drive, even a set of headphones I tested has a power brick for the pre-amp. In any case the average user probably has six power bricks laying around, and most of them would be lucky to get 80% power efficiency ratings... just hold them and feel how warm they are, for a tiny device like a cable modem they can get quite warm indeed. Trashing all of those in favor of a single high-efficiency PSU and USB network is appealing in some ways, although perhaps not easy to implement and not practical for most.
Given how my desk is set up though, it would be easy to run a USB cable to my monitor (my monitor already has a USB hub built in, many do), and since my monitor is next to the printer, modem, and router I could very easily connect them all off one daisy chain. Apple is already doing this with Thunderbolt and their monitors / devices...
