In my experience carbon forks are better at muting high-frequency vibrations, such as the ones on my old Cannondale R400. In other words when you're on rough pavement at 20 mph they help to reduce harshness. Carbon has poor resistance to direct impacts, such as a hammer blow... or a low branch... or evil squirrels' teeth.
Personally my off road machines will never use a carbon frame, fork, handlebar or seatpost and I'd certainly not use carbon components with the sole intent to make the bike lighter since it's cheaper and easier to take off the weight from around our waists.