First of all, open carry is letting the bad guys know what you have and they will take measures to avoid it or just shoot without warning, that's why I wouldn't do it, never said that other shouldn't be allowed.
Secondly and I think more important Is the bad PR it gives gun owners with folks who might be on the fence. That housewife who doesn't know which way to come down on the gun issue and then sees some guy walking into the supermarket with an AK-47 or AR on there back looks really bad. We are in a fight to keep these rights and we as a group need to take a responsible approach and not freak out regular citizens who we will need on our side. If the Dems get Clinton in office they will end up with the senate and maybe even the house and we can kiss our guns goodbye. They will ban everything under the sun with the exception of single shot firearms.