Although I now live in Alabama I lived for 26 years in the Great State of Texas. (I miss TX ) During my years there I learned of two incidences where LEOs were shot and killed by armed robbers while in stores.
In the first instance a plain clothes Deputy Constable was visiting a friend who owned and ran a jewelry store. This LEO was known for "flashing", deliberately allowing his gun to be seen. While talking to his friend on the customer's side of the counter an armed robber came in, saw the gun, fired and killed the constable before he had a chance to respond. It is said that the only reason others in the store were not killed was because the store owner then retrieved his hidden gun an drove away the killer with it.
In the second incidence a plains clothes HPD officer was writing a ticket for a clerk in an adult book shop when an armed robber came in. The officer's gun was concealed but he was wearing a raid jacket with "POLICE, HPD" or something to the effect and was gunned down before also having the chance to defend himself.
On the other hand in the July/August issue of American Handgunner ( At least I think it was that issue. Either that or the one before it. ( I threw it away already)) Mas Ayoob recounts the case where a CCW citizen was taken by surprise by two armed robbers in a sandwich shop. At first the citizen complied with the robbers' demands and gave them his wallet. Then the robbers ordered him and the clerk, the only two others there, into the back room. The citizen realized he was about to be executed so he "went for it" as he had no other alternatives. He succeeded in killing one of the robbers and wounding the other without being injured himself. He would not have been able to do this if he had been carry openly.
There are many more similar stories out there. Because of these incidences I will not normally carry openly. However, that does not mean that open carry should not be allowed.