Total BS. Try riding through a fast, bumpy, technical section of singletrack on bmx pedals and see how well they "lock" your foot down. You will fail and there will be pain involved. Clipless or at least clips and straps will keep your feet on the pedals a tousand times better than a bmx pedal ever could. And I know your argument will be "Bmx riders use them and there feet don't come off the pedals!". Watch a Bmx race, they pedal hard through the smoother sections and then stop pedaling over the bumps. Do this in mountain biking and again, you will fail and there will be pain involved.