your legs won't rock or swing any more than it does with self-centering pedals if you have proper muscle build up.

your knees will never go up and down straight, so it's good to have some float and it's also good for your knees if you build up muscle to keep your leg from swaying in odd directions.

This goes contrary to a recent visit here by John Howard. The very first thing John did was to tighten down the float on nearly every pedal. Float isn't always a good thing.