Case in point: My WAR sorcerer had an area-of-effect root spell. It doesn't always root every enemy around me to their position, but it has a good chance to. Anyway, I found three uses for that one skill just within an hour of RvR.
- stop an injured enemy from retreating to safety
- save an ally from pursuing enemies
- delay a group of enemies just long enough for my allies to regroup or reach a more defensible position