I always carrying extra batteries in my camera bag so if they die, I still have extras. A few months ago I was at Huntington Beach State Park taking photos of wading birds (egrets, herons). Deciding against carrying my heavy camera bag, I just grabbed one of my cameras, my extra memory cards and headed to take more photos. I was on the other side of the causeway from my car when my batteries died. As I was walking back to my car to replace them I heard crunching near me. I turned to look and not five feet in front of me was an alligator facing me munching on a blue crab. It would have been a great shot but because I hadn’t brought my extra batteries, I did not get the shot.
A few weeks later I was sitting at a light when to my right I saw a Red Tailed Hawk 10 feet away standing on the ground. Because I was heading to a meeting I had not thought to bring my cameras with me.
No batteries or no camera = no shot. The moral of the story, just like the boy scouts, is "always be prepared." You never know when you perfect shot will be presented to you and you want to be ready.