Well, I'm cocky enough to think I have enough skill to 'jib' off features along the trail 9/10 totally fine, 1/10 knees take the punishment. So in the interest of protecting my future riding I'll pad up.
Edit. Not XC as in race, local trails, plenty to keep you on your toes or knees...
What's the point of the knee guards for XC? What are they protecting against? Do you land on your knees often?