Sharpshooter:
Back in the days of the dump pouch, handing a fellow Officer a few rounds, although a significant portion of your carry load, probably wouldn't have made as much a mess as today, where the guy with two spare mags is giving up half his supply, perhaps....
I used to carry two dump pouches and twelve FMJ's in loops on the rear of my belt - they looked really nice, but weren't intended to be shot. The PD didn't want to hear about .357's at all. They eventually issued stainless M19's - I forget the model number - but still wouldn't let anybody carry the .357's. Hollow points were Politically Incorrect, too!
In the years that I was closely associated with YPD, I don't think the entire PD fired more than ten rounds "in anger" from a handgun, including two or three riots. ('Course, our riots were pretty tame....)
Not that you couldn't share ammunition, but it's got to be a more carefully thought out decision.
Then there's the magazine interchangeability issue, too.... The M&P40FS and M&P40C can use the longer full size magazine. It means that your BUG can use your standard magazines if necessary. Within the "you should always have a spare magazine for anything" view, it's an advantage. Two full size magazines on your belt, one in the duty gun, and one (small) magazine in the BUG, and you're well set up. If the stuff really hits the fan and you need to clear a problem with the BUG, you may still have a spare mag for it, without having to hassle an odd magazine.
Regards,