From what I have heard the fuses only work when the tires are straight, whether this is truth or fiction I am not sure. Now if it is true than it is still worth the few bucks and easy replacement than what could have happened...changing a CV shaft(if you have one) on the trail is not something any of us want to have to do.
PS I read an article about the hub fuses and lockers, supposedly a big no-no...I believe it was in one of the recent 4 Wheeler mags.