Referencing the duster boy (and other pics), it's not even just the costume-like apparel some people wear, but how they carry their firearms.
I don't know Duster Boy. Maybe he thought he was being safe and following the four rules. However, I always try to look at this stuff from the side of the people we are trying to persuade. If I want to convince people who know nothing about guns that they have nothing to fear, do I really want to walk around with firearms in a "ready" position, or even in a way that looks like I'm in a "ready" position?
A lot of these people walk around like they are in Condition Orange. If I'm already leery of guns in general, the appearance of "shooter ready" doesn't make me sympathetic.
I know that in some places, open carry demonstrations have to be done with long guns, because you can't do so with pistols (still the case in Texas?). However, the best "advertising" for open carry to me is the person who walks into the McDonalds with an open carry holstered pistol (and not holstered in a thigh rig) to order the #3, eats his or her lunch, and walks out, just like any other person - no muss, no fuss. The gun is no different than a cell phone in a belt holster.