I don't know about you but in the past when I was being shot at I had tendency to find cover soonest, even when wearing a flak vest. Those around me did the same thing. It takes a great deal of training a practice and to run in the direction of the gunfire. There are times when that is your safest option. I would bet all I have Holmes did not have the training nor the practice nor would he have had the need someone was returning fire. What he would have done was seek some measure of concealment. Get my point? It is damned hard and usually stupid to do otherwise, I don't care how much armor you are wearing.
Point 1. If someone had returned fire, Holmes would have stopped his movement other than to initially seek concealment. Believe it. He would also have had to address the person(s) shooting. It is logical to assume that others might have been able to get out of the theater while Holmes attention was focused elsewhere.
Pistols, Revolvers come in all different shapes and sizes. Some people carry .45 cal pistols, some carry .380s and so forth. If one or more people were returning fire it isn't likely that Holmes would know exactly what people were shooting at him.
Point 2. While a .45 caliber bullet would probably not penetrate his armor, if the bullet hit him it would hurt like hell and could knock the breath out of him. At that point he'd know he was getting hit with something big. He would have to act accordingly at that point. That's another plus for the good guys. A big plus, we don't know. Anything is better than nothing and more is better than less.
Point 3. Body armor is never perfect. People get shot in weird places like armpits, through the nose and in the seams of body armor. Holmes was/is probably insane, but he isn't reported to be a dolt. If he had been hit with one round it is very unlikely that he'd just stand there and fire away.
Other people having guns in the theater may have very well mattered.