General rule of thumb for backup: If you don't have you wish you did.
It's not just equipment failure, it includes theft, dropping the camera or lens, a tripod lifting a leg, loosing a CF card... the list goes on.
For each of the last ten years, I've had at least one card go bad (but nearly all the images recovered), batteries die at the wrong time, dropped lenses, had tripods go over, lenses lock up and more. It isn't a question of if, it's always when and "when" is usually when you are the least prepared...
Three years ago, I was shooting an annual cattle drive in the high Sierra Range. Though I have multiple lenses covering the 70-200mm range, this day I simply took one.
At the second place I started shooting, the lens locked-up focus at 6 feet. I had wider and I had longer. At this point, the needed lens was at that focal length. As I drop to the next point to shoot, I started taking the lens apart to see if I could coax it back to working. I was lucky and pulled it off, but it's tough to drive, fix a lens and get to the next photo site all at once...Had I simply put my 80-200 in the car with me...