You know, it's entirely possible that a life-or-death fight might disable your dominant hand.
But really, it's much more likely that more normal activities might disable your dominant hand, prior to a life-or-death encounter.
The most common injuries in skiing, snowboarding, skating, rollerblading... are hand and wrist injuries. People tend to break falls with their hands, yet hands and wrists are made of lots of small, relatively weak bones and joints. They are very easily disabled.
Yet another reason why it's a good idea to be proficient with either hand. Don't wait until your dominant hand is in a cast, or dominant arm in a sling, to figure out how to use the weak hand.
I try to put 20-25% of my training time into weak-hand shooting. I'm almost as accurate with my left hand as I am with my right, but I am somewhat slower.