Its hard to define just how useful the ability is until you use it and see how many different ways you can use it. As stated, the biggest benefits are to tanks and healers (and *gasp* paladins who sometimes find ourselves doing both).
The most obvious advantage is that you can see when your target has switched targets. This is quite often a trigger for you to do something different (in so many different ways that we could list them all day long), so it's pretty useful to know all by itself.
Being able to easily see both the targets health and his targets health allows a healer to see both the tank and the mobs health at the same time, regardless of who they are targeting at the moment. Obviously, if I *am* the tank, and that switches, I know that I need to do something to regain agro. If I'm not the tank, I may drop a stun on the mob to save some squishy who just got agroed, perhaps followed up with a heal for the poor sap. It's just nice in general since you can judge when to cast heal spells, and when you don't have to. Yes. You can see your group's health in the group box, but you "lose track" of the mob when you target them.
Druids/shamans as dual nuker/healers can more easily fill both roles. Again. The benefit of being able to see both the mobs health *and* the health of whomever that mob is beating on at the same time is pretty darn useful.
In pretty much all cases, it takes a bit of the mystery out of /assist targetting as well. While some roles don't care as much (dps class is just going to assist the MA and dps), anyone who needs to retarget regularly througout the fight will find this handy. The possible ways in which it's helpful are numerous...
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King Nobby wrote:
More words please