I'm going to try to build a sub-15-pound fixed-gear bike in the near future, so we'll see how that goes. The reason you see so many 15-pound derailer bikes is because 14.99 pounds is the lower limit for UCI races, and the people that spend money on light bikes are doing that to ride them in UCI races.
As for wasted power; I picked a gear that matches my fixed gear bike's gear and rode both for a mile at a 95rpm cadence. Same heart rate both times, which means my body is working equally hard to propel both the same distance and speed. Pushing your sail-like body through the air is where your energy goes when riding a bike. Everything else is a rounding error.
(It would be nice if someone with power-measuring pedals and a power-measuring hub could try both scenarios, though. Then you would know exactly how many watts are being lost in the drivetrain.)