It's only common sense

[onecaliberal]

That open carry would create chaos for police in a situation like Dallas. The "good guys with guns" ran for safety with the rest of the crowd, which only made it increasingly difficult for police to determine how many and who exactly was shooting. Definitely destroyed the open carry argument in every way shape form and fashion.
Edit to add: What could open carry advocates possibly still argue in favor of?

[Matrosov]

1. To a certain extent, it goes for concealed carry as well

Imagine there's an active shooting at a mall. Most concealed carries would likely be more interested in getting out of there as quickly as possible and wouldn't draw their weapon unless they're confronted by the shooter. Nobody else would know they're armed and it wouldn't be a problem.

But what if one is wanting to be that 'good guy with a gun,' heads towards the gun fire, encounters someone else with their weapon drawn, but doesn't realize the other person isn't the one shooting innocent people? That could even happen with law enforcement running into the 'good guy with a gun' with his weapon drawn and in the heat of the moment the police deciding to shoot first and ask questions later.

Eventually someone thinking they're helping by being armed is going to get shot and killed in a moment of confusion.

[onecaliberal]

2. You've explained the exact crux of the problem. In an active shooter situation

It's a chaotic situation, police are trained to go to the shooting and take out the threat. It is entirely possible that someone trying to be helpful could find themselves the target of police not knowing they're one of those "good guys with a gun". It's just a bad idea all the way around.

[Igel]

3. Many things require common sense.

A reasonable use of most rights requires common sense. With every right comes an obligation, both on others to provide the right if it's one that needs group participation as well as on the part of the user to use it reasonably.

Free speech is fine. But there are not only legal limits on speech but common sense limits on speech.

Freedom of religion is fine. But there are not only legal limits on it--human sacrifice is right out--but reasonable limits on what you can expect others to put up with. I keep a sunset-to-sunset Saturday Sabbath. I keep vaguely kosher. When visiting colleagues, I did not assume that they had to cater to my dietary requirements. I get a slab o' pig about to descend on my plate I beg off and risk offending my host by saying I'm not really hungry. Then I pick at the salad or the bread. I don't assume every employer I may apply for a job to meets my Sabbath requirements. Nor do I go ballistic when somebody swears in front of me. One has to be reasonable.

Open carry has common-sense restrictions, not just legal restrictions. The same attitude that makes free speech, etc., go horribly wrong makes open carry go wrong. I go up to a cop trying to arrest me and give him a hard time, calling him all sorts of names he's going to be pissed off. There's a lack of common sense, and it's like flashing $10000 on the street in a rough part of town at midnight while drunk and about to pass out. Yeah, I have a right to my personal space and not being assaulted or robbed, but if I'm foolish then I'm a fool. My right has a personal obligation.

So you take open carry to a protest that has a police escort and let the police know what's up. Why? Because you're somehow their lickspittle? No, because it's common sense if something goes wrong. You want them to know you, know what's up, know where you are.

The guy in Dallas who was a person of interest was treated reasonably because after he did something lacking common sense he then used common sense to repair the damage. He wasn't shot, as many would have predicted, just for open carry. He wasn't wrestled to the ground, beaten senseless, and killed, as many would have predicted. He used common sense and all was cleared up in the amount of time you'd suspect.

The problem is showing that you have a right by using it in a senseless manner. I've known people who open carried, and they did it when there was a reason other than, "See, I have this right, I dare you to stop me." That's foolish bravado. Lots of people are fools. You watch videos of escalating police situations, and you see lots of people using their rights just to show that they can, regardless of the consequences. All rights, no obligations. That's foolishness. It's like flashing $10000 in a rough neighborhood while drunk. Yeah, I have a right. Whoop-dee-do.

And those are just rights that require nothing but forbearance and restraint on the part of others.

[onecaliberal]

4. That person of interest was far from being the only one to open carrying that night.

It was a monumental disaster for police to determine if they were also shooters. How were police to know which of them were good guys or there to kill officers? Resources were unnecessarily expended to make that determination at a time full resource were needed to contain the situation at hand. It was the last thing the police needed. Those open carry people ran away like everyone else did. Then had to be tracked down to see if they were shooters who slipped away amid the chaos. You might think that's a good use of resources in that situation, I on the other hand view it as the disaster the police department determined it to be.
Most of the other rights we enjoy do not entail endangering the lives of others. Open carry is a epic fail!.

[Skittles]

5. all gun humpers care about is THEIR GUNS

they DO NOT CARE about what happens because of their insane obsession