If it gets turned over to a Grand Jury, your spare ammo will be used against you. In the event you are not familar with Grand Jury, only the prosecution side is presented. Your defense atty or yourself will not be present and your side will not be heard. Having a box of shells on you will not go in your favor. From there, things will get costly for you.
I never have bought into this line of thinking. A good shoot is a good shoot. Carrying spare ammo is not going to make a good shoot into a bad one. Besides, the grand jury means nothing. A grand jury can indict a ham sandwich. Since you cannot present your (defense ) case, it won't matter if the only bullet you had was the one you put in a serial rapist's brainpan while he was stabbing his latest victim, because if you get a DA that is scummy enough to use a spare mag against you, he will be scummy enough to make sure that all the jury will hear is that the man you shot went to church on Sunday and helped little old ladies across the street, and you shot him because you were a gun nut and carried a gun with you everywhere you went, because you were a paranoid right wing wannabe Rambo vigilante. Even without a spare mag, he will try to paint you to be the bad guy, if he is so inclined.
Your side comes out in trial. That is where you raise your defense.
BTW: You claim that you fire 5,000-8,000 rounds a month, and you have been a LEO for 38 years and have never experienced a magazine failure, and neither have your uncle or cousins. A total of 118 years of shooting between you at 5,000 rounds a month. Doing the math (5,000*12*118)= about 7 to 11 million rounds fired without a single mag failure? That is pure BS.