Open carry can be argued as a deterrant. You'll not generally find hard facts to tell this, but lots of folks theorize that if someone is thinking about robbing a gas station and see someone inside already armed, they'll think twice. Opponents to OC say that the bad guy will just shoot the armed person first, but there's not really any verifiable evidence to prove this idea.
I don't buy the opponents argument that the BG will just shoot the OC'r. This is just an opinion, but I think most guys robbing a gas station would not bring on a murder charge if they can just wait a couple of minutes until the OC is gone. That would only apply if the BG saw the gun before making his/her move.
Now, if they are in the midst of a hold up and then see the OC, then who knows how they will react. Hopefully, before they can react, the OC guy is now the guy with the gun pointed at the BG and has the initiative.