Unless you're touring or doing long randonneuring rides, there usually no reason to have to carry a fiber fix spoke. In an emergency, for most riding you can survive a broken spoke by opening up the rear brake and adjusting the tension on the other spokes to true up the wheel enough to not rub against the frame. If you have a low spoke count wheel, this may be difficult, but otherwise, you should be able to get home without resorting to a fiber fix spoke.

Obviously, if breaking spokes has become a habit, it's time to have the wheel rebuilt. It's cheaper to do the whole wheel at once than a spoke at a time.