It is rare. I have heard of a few occasions where an LEO had his gun used against him.
I carry concealed during the winter months when coats are needed and open during the summer. All my holsters have thumb straps.
I fully agree with the recommendations to maintain situational awareness (armed or not), use a holster that impedes the snatch-and-grab and to practice weapon retention. But I also agree that most criminals who see that a man is armed will move on to greener pastures.
Oh, and you don't need an Eastwood scowl (or a gun) to look like someone who is not good to mess with. Stand up straight, look people in the eye, look like you know what you're doing (even if you don't) and don't say much. ;-)
(OK, I just described Clint Eastwood didn't I?)