As I understand it, it's a sustained ability. So when it's on, you get a little bit less mana to work with, as some of your mana's reserved for that electric weapon. IMO, whether it's useful or not largely depends on your choice of class. Rogues are hit-and-run, and Warriors are about putting your enemy down FAST, so it works well for both of them; every little bit helps. But Guardians are about tanking and outlasting enemies, so it's not as important to them; don't need to deal more damage when you can handle anything thrown at you.