The knees and lower legs are VERY prone to damage. Don't forget the front of the leg which has little or no protection over the Tibia.
Spending a lot of time climbing over seaside rocks I take no chances (I already have legs full of Titanium "Motorcycles") and use full knee and lower leg protection. http://www.anythingxtreme.com/Leg-Kn...ion-C2098.aspx
Another advantage is that if you trip you can fall on your knees without injury and avoid discarding that $8000 asset that you are carrying in your hands.