I'm sorry Neil, but as an athlete, this is and has been one of the most misleading statements in pool on the internet for 15+ years. The coordination of more muscles is the definition of advancement in athletics. Or, to say it better, coordinating the right number of muscles to make the most efficient use of the motions. There isn't a single physical activity that differentiates excellence or even decent versus also-rans that violates this. Pool has never been different regardless of all the feel good sayings. It seems like only those who don't know a thing about athletics or think about pool as a physical endeavor say things like "less muscles are better" or some such thing in pool.
Even the dart professionals move their arms and elbows all over the place! Why? Because it would hurt if they kept trying to keep their elbow still.
The idea of the dropped elbow is at its simplest a method to not retard your motion. It takes more effort to not drop your elbow on that full out stroke. I completely agree that keeping the elbow from dropping leads to good repeatable stroke on most average shots. But what differentiates excellence versus average is the above average shots and the ability to make the average shot even easier, as coordinated excellence affords. That's why pros are pros.
I'm sure everyone here is an athlete now, so everyone can give me their feel good story. But I'm an athlete and have been a coach. The study of motion in pool is no different.
I'm not an athelete but I have played one on T.V.