In 1983 I was involved in just such a situation. We were carrying 439s at the time. The issued mag pouch was for a single mag but we could carry double pouches if we bought our own. Another Troop was involved in a shooting against 2 individuals. When I got there he had already fired both of his mags. One subject was down and the other was armed and on foot. When I got there we could have waited until he reloaded his one mag. It was about 2 AM and his first mag was on the ground in the dark so it was out of play. It was a lot quicker to toss him one of mine and we went after the 2nd subject. Not tossing him a mag would have been a stupid move.The least convincing, IMHO, is the "compatible ammo" theory. This theory states that in a crunch, an officer low on ammo can obtain extra ammo from another officer at the scene. First, there are seldom those kinds of extended firefights. Second, if an officer has already used up his own supply of ammo and there is still a serious shoot-out going on, you question the effectiveness of his shooting.
The other argument often heard is "If he shoots up all his ammo then why should I trust him with some of mine?" People who say that don't think very deeply thru a situation and show a lack of real world experience in such situations. Think about it. Your partner and you both engage a subject who is returning fire. With both firing the subject has 2 threats facing him from different angles. If one of those firing is no longer a threat then the subject only has to worry about the remaining person firing his way. That means if your partner is dry and no longer firing but you're still firing, guess who is the focus of the returning in-coming fire. If people still have difficulty figuring it out, turn the tables and think about it. 2 people are firing at you and 1 goes empty and quits firing. Would you rather the bad guy with the ammo be the only one still firing at you or would you rather him toss his bad guy partner additional ammo so the 2 of them can engage you?
Just because your partner ran dry does not say anything about his shooting ability or his lack of control. I was at one shooting situation where I was only 5 ft away from another officer. I never could see the bad guy from my location but the other officer had a clear view and could engage. Had the bad guy moved just a couple of feet in either direction I would have had a clear view but the other officer would have lost sight of him. Now had the other officer run dry would it have been a smart move to tell him "You shot up all your ammo - heck if you think I'm giving you any."