here is a reason I like being clipped in that I dont think others have brought up--I find being able to change up my pedal stroke--muscles wise--is a plus for changing up how your leg muscles work, and so gives a slightly diff muscle group use at times which feels nice.
I notice that I instinctively change up how I pedal during the day, and I think it helps with overall comfort during a long day.
+1 And more - I've had injuries and knee issues that were made worse during the downstroke portion of the pedal cycle. I have ridden many miles simply "deleting" the downstroke, ie relaxing as my foot comes over the top and staying relaxed until I can drag back at the bottom. Power up. (And yes, I was taught very well the bad habit of "pedaling circles" and educated my leg muscles on relaxation by riding down big hills on fix gears. I know that bit about the upstroke is physically impossible (edit: thanks forum; learned that here)
but it has bailed me out many times.)
Oh, that knee injury is my chronic issue of CP, chrondomalcia patellae that I've lived with the past 40 years. Sometimes I ride when I haven't done my homework and it surfaces. Having a tool to minimize it and keep riding is really useful.
Ben