Except in the case of an OC'd firearm because 90% of the usefulness of an OC'd firearm is in deterrance rather than defense. To put it bluntly, only the dumbest of criminals would still consider robbing the Taco Bell while SovereignAxe was there OC'ing. Once a criminal sees someone OC'ing they know the place is a hard target and they prefer soft targets. Even if the OC'd firearm is unloaded and the person OC'ing doesn't even have ammo for it, the OC'd firearm is able to carry out it's full use as a deterrant. It only loses it's usefulness as a defense mechansim. However, if the weapon does it's job effectively as a deterrant, then there is no need to use it in defense.
To put this in different terms, mutally assured destruction (MAD) prevents wars from starting because it acts as a deterrance factor. The only thing required for MAD to work is for each side to BELIEVE that the other side has the ability to wipe them out. In fact if a nation did away with their nuclear arms altogether, but the other side didn't know that, MAD would still work as a deterrance factor.
That's what makes OC'ing such a valuable asset to the entire community.