I carry because I went through all the hoops to get a card that said I could. I took a class, had to demonstrate proper firearm control, I was fingerprinted, profiled and background checked for the FBI and they've got all that on file with the State Police. I also had to go in front of the gun board along with pay some hefty fees.
Fact of the matter is, I never ever want to use it.
Working with amateur radio, it's very possible sometime I will be involved with a disaster event. The cops 'securing' the place aren't going to do anything to save me if someone crazy shows up. They're going to be looking out for #1 and that's it.
I also used to work third shift and spent a fair amount of time alone in buildings here at work with millions upon millions of dollars worth of equipment.
Also, having one of those cards requires you to disclose to a peace officer immediately, which is in of itself a way of telling the officer that not only have I been fingerprinted, I've had a background check and have a completely clean record (anything over a speeding ticket disqualifies you in MI). They tend to believe what you tell them.