"Why don't you just lock the bolt to the rear BEFORE yonu insert the mag?"
Very simple. You can't always choose exactly what course of events is going to develop, therefore you must try to be as prepared as you can for whatever might happen.
If you always shoot your AR15 on a shooting range where you can decide every move you are going to make then sure, lock the bolt back. Load as many rounds as you want. The paper target is definitely going to be there when you get everything just so. Maybe you have no interest at all in defensive type training with AR15s.
Certainly, do whatever makes you happy. Follow your own experience.
In the defensive type shooting sports and defensive type training, there is a concept called the "tactical reload". This is a very common and widely practiced drill. I have never been in any kind of combat but it seems logical that being able to perform a tactical reload smoothly and efficently would be of great value. I have participated in numerous shooting sports and drills that require you to do a tactical reload while on the clock: so, successfully passing that phase of a class or scoring high in a match demands that you can do a quick and efficent tactical reload.
The concept of the tactical reload is that you have fired your weapon. The threat has gone away for the moment (the enemy is down and out of the fight, the enemy has left the area, whatever). You are now holding a weapon with an unknown number of rounds in it. You don't know what the future holds. More bad guys may materialize in the next instant; you simply don't know. Ideally, you want the weapon loaded to it's full capacity because you don't know what might happen in the future so you want to be as prepared as you can be. So, during this lull in the gunfight, you quickly and efficently remove the partially depleted magazine and insert a fully loaded magazine. It is also highly desireable to retain the partially depleted magazine if you can but that is another subject. You want to spend as little time as possible with the magazine out of the gun and as much time as possible with a fully loaded rifle, ready to go. That is a tactical reload. If your rifle won't accept the magazine you just tryed to insert, now you have to do something about it. You could try to pound on it, you could cycle the action and lock back the bolt: OR YOU COULD JUST LOAD YOUR MAGAZINES WITH 28 ROUNDS AND NOT HAVE THE PROBLEM AT ALL. Why ? Because anything other than smooth and efficent is not good. What if you come under attack again while you are screwing around with the magazine because you absolutely insisted that you were going to put 30 rounds in it ? What if you came under attack and didn't even know it because your focus was on your rifle and magazine ? What if you were simply involved in a shooting game where you are being timed for the senario and the senario requires you to do a tactical reload ? Instead of quickly finishing the stage or match you are wasting valuable time screwing around with doing a reload because you just couldn't possibly live with the idea of downloading your magazines by two rounds ?
Certainly, your milage may vary.
But, for the life of me, I can't understand why this is such a big issue. I can't imagine why people who appearently don't really care about making quick reloads are so concerned with having the rifle hold those two more rounds. It seems as if some people are willing to do anything in order to have those two rounds. They will debate endlessly about those two rounds. Those two rounds are of paramount importance..................but, quickly and efficently charging the rifle with a fresh 28 more new rounds isn't even on their radar screen.