Very interesting discussion. I would like to throw my two cents into the action.
First, as a point of information for those that have never been in a firefight, when that times comes (and we all hope it never does come) you WILL revert to your training and what some term muscle memory.
My former Agency trained for CQB (close quarters combat). Our guns were never out of our possession except when we were in the shower or sound asleep. Our training dictated that when we strapped our gun on after it was off our body, we did a mag check to insure it was properly seated and a press check to make sure a round was in the chamber. It cost us nothing but a second or two of our time.
We also trained for two types of reloads: Emergency and Tactical. The emergency reload was when the slide locked back. The magazine was ejected and a fresh mag was inserted and the slide returned to battery. I always try to count my rounds while firing but in the heat of battle with a 12 or 15 round magazine, it is not always the easiest thing to do. The tactical reload was when some rounds were expended and there was a lull in the action. The partially used magazine is ejected and held while a fully loaded magazine is inserted into the mag well. There is still a round in the chamber and you still have a magazine in your possession that has rounds remaining.
Training was always with two spare magazines. Today, many years later, I carry two full magazines with me when I carry. If my gun is not on my body, it is because I am in the shower or asleep. However, it is close by as is the two spare magazines. And the press check, I do it every time the gun goes back. For me, its just a little peace of mind...a little assurance that everything as is it should be.
May the Schwartz Be With You.