First of all, the efficiency difference vs. Powergrips or toe clip-strap pedals is pretty small. Those also lock your foot in pretty well and let you spin. When people say "clipless pedals are a huge gain" they are comparing to platform pedals which is fucking ridiculous, you need to compare vs. toe clips. The nice thing about toe clips is you can leave them loose around town, and then when you get out on a long course, you just reach down and pull the strap tight and then you are locked in nice and neat.
The first problem with clipless pedals is ergonomic. Yes, they can be adjusted just right so that you have good geometry and they won't cause any pain - but that is a big pain in the ass, and the average amateur doesn't have the perfect adjustment. The result is knee and hip pain. The extra freedom of strap pedals lets you get a more comfortable position and avoid the pain.
The biggest problem with clipless pedals is that it turns the amateur road rider into a real dick-head. They aren't comfortable with clipping in and out, so they go to great measures to avoid it. They'll hang on to posts at red lights, run lights and stop signs, won't wait in a line of other cyclists, etc. They create a real hazard because they can't get their feet out easily.
Of all the dick cyclists on the road, the yuppie amateur road racer has to be the worst. They're the ones who are all wobbly and don't stop for pedestrians. They ride in big groups and don't get out of the way for cars. They run stop signs and act like they aren't doing anything wrong. They'll often ride way out in the road for no good reason and not get over to let cars by. Of course cyclists should take the lane when they need to for safety reasons, but that is not the case for these turds.
It really makes me sad when I see some out of shape people who are obviously trying to get into cycling, and the fucking shop has set them up with some harsh aluminum frame, with a way too aggressive forward position, and clipless pedals; they're obviously very uncomfortable on their bike, and also out of control.