I find this excuse to be pure BS 99% of the time, whether it's a Glock, M&P, or otherwise that is malfing. It's usually the gun, unless the shooter is like 98 years old and has the hand strength of a toddler, limp wristing is improbable most of the time. If I want to purposely limp wrist a gun (eg, to -make- it malf) I have to hold it so loosely it'd be considered a dangerous condition by most. There are some exceptions here and there- eg, POSes like the Seecamp .32, etc, where if you don't death grip them, they will malf, but for something like an M&P.... no way.
-Mike
Same here. I've tried on all of mine without succeeding.