Well, gun owners tend to be quiet and respectable people. That partly because getting arrested (or committed) will pretty much lose you your right to own a handgun.
Also, it's not a great idea to advertise that you have guns, a decal on the window: "Protected by Smith and Wesson" is an also an invitation to a break-in when you are not at home.
As far as having people know if you are carrying - the last thing you need to do is to run into a hyped-up adrenelin-overloded cop who thinks that his best chance of going home alive tonight is to shoot you first.
I'm a big proponent of Open Carry. Make tooled leather and fancy scrollwork on a pistola a fashion statement again. Yeah, the fainting lillies and whining Sucker Moms will urinate on themselves... But their blind bias should never be enough excuse to disarm the "Good Guys".
As for "hyped-up" cops, these days... it seems there are plenty that'll shoot you no matter what you do. And yet, I'd still go to an officers aid in a bad situation. I'd like to thhink that'd engender a certain spirit of reciprocity.