Having a cannon instead of nothing gives you possibilities, like having a Swiss army knife in a jungle survival scenario. Yes you're probably going to die, but

Having a cannon instead of nothing gives you possibilities, like having a Swiss army knife in a jungle survival scenario. Yes you're probably going to die, but at least you are not a sitting duck. They even give pilots a pistol or a sub-machine gun in case they eject over enemy territory. It's probably dead weight, but if it can save someone once, why not? I wouldn't go without it even if intellectually I know I'm not going to take down the whole enemy army with a 15 shots pistol.