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Imagine RvR whereby structures such as walls and seige equipment can only be built by crafters. An RvR group would almost always want to bring along some crafters to build the seige equipment or defensive structures or defensive seige equipment to improve their odds in RvR.
Thus, if you had crafters in your group, you could build a defensive tower in the middle of enemy territory, greatly enhancing your ability to survive attack. The more crafters, the faster and larger that defensive tower could be, up to a full-blown defensive fort with multiple walls. Conversely, attacking these defended positions would require groups to bring crafters along to build the seige weapons needed.
In this regard, crafters become an essential support class for RvR. In addition to making swords, armors, etc. they can also be active in the RvR game.