It's very common. Most lefty shooters I know prefer to use the right-handed mag release; they find it faster to use their index finger. Heck, I've even seen some right-handed competition shooters switch their mag release to the left-handed mode so they can use their index finger.
I do that as stated above. Takes 10 seconds to switch a M&P mag release, and my short fingers, and having learned to shoot on a USP makes using my index finger generally more comfortable for me, also instills keeping your finger off the trigger until back on your target.