You're on vacation, and you've got your trusty digital camera with you. You go to snap a once in a lifetime pic, and then you get the message "Card full".
You could panic, but you've got a spare card with you, just in case of emergencies. Pretty smart.
So what else do you carry around in that camera bag of yours besides the camera?
I don't carry a camera bag - too bulky. I do carry my laptop with me so that I can download my pics to it in case I run out of memory on my card. And definitely have the extra battery too!