You're almost right. We were taught to do a tactical reload when possible and when from cover. So if you're crouched behind a car and you have 5 rounds left and want to do a mag change, stow the mag in case it is needed later. If the bullets are whizzing past your head, of course just drop the magazine.
While I've never been in a shootout, I know guys who have, and NONE of them thought about dropping a magazine that still had rounds in it. How many rounds do you leave on the ground? Do a mag change with 3 rounds left, or with 8? How about 10? That's a lot of ammo to leave behind. That's also assuming you have the presence of mind to be counting your shots, and I don't know anyone who did. One cop I worked with got into a shooting and he emptied his gun at the bad guy, hitting him several times. When we showed up, he was covering the wounded perp with an empty gun, slide locked back. He wasn't even aware.
Dropping rounds that I may need later just seems to me to be not such a great idea, but every situation is different, I know. So if you drop a partially loaded magazine and things are too hectic to maintain the magazine, then drop away. But if you fire several rounds and then have to give pursuit, I would say keep the partially loaded magazine. But that's my take. I don't even carry a spare mag now that I'm no longer on the job, so I don't worry too much about it.
Your experience certainly dwarfs mine, so I'm not criticizing. Just seems to be not such a good idea to leave ammo laying on the ground that you might need later. Doing a tactical reload at all means things are going crazy. How much crazier they get is anybody's guess.