As others have stated, the magazine safety can be added or removed by an armor, no big deal.

I'm not a huge fan of the magazine safety, but with that said, my agency has two documented saves where during weapon retention incidents in which suspects were trying to disarm the officers, the officers managed to drop the magazine as the gun was being forced out of the holster. In both cases, the suspects attempted to shoot the officer with their gun, only to have the gun not function due to the magazine safety. Both of these cases were pre - M&P, when the issued guns were 5906's and then 5946's.

The nice thing about the design of the magazine safety on the M&P is that it's just a few parts and a couple of minutes. Put it on, take it off, your choice.