Open carry also makes false complaints of brandishing or threats easier to make.

I former professional acquaintance of mine was once accused of pointing a gun at someone. When the police arrived, the "victim" was asked to make a statement in which he described the firearm involved as a black revolver. The acquaintance was carrying a silver semi-auto. The "victim" admitted that he made the story up after seeing the NRA sticker on the acquaintance's car and being told by a mutual friend that he had a carry permit.

Had he been openly carrying, the person making the false complaint would have been able to describe the weapon and my acquaintance would have been in serious trouble.

Note that the converse works as well - if someone with a concealed weapon pulls it, witnesses would be able to describe it accurately which would tend to indicate that they actually saw it.

Matt