You never did stumble on one of these tanks who keeps mobs so far from each other that you are indeed better be single targeting rather than aoeing ? XD
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I usually switch targets cause usually a skill in my rotation has AOE damage attached to it(Foul/Bioblaster/Auto Crossbow) and I know it'll die to that.
In some situations (like a boss fight), it might be wiser to switch to the adds with full (but still has lower hp) than the nearly dead (but still has higher HP). And sometimes you just want to kill something ASAP, so single target might be preferable to AOE.
Dots normally can kill the enemie once it reachs 5% so I'll switch to another so I can fully focus on it if it has Dots on it.
Pretty much this for me as well. If I know the other DPS in the party has better AoE than I do and the tank is likely to continue AoEing, or there's a DoT on the almost dead enemy, or there's a WHM who is Holy casting. I've been running a lot of SAM recently and I'm not going to blow my cooldowns on something that's going to die in seconds from what the other party members are doing better than I can anyway.
And yet I will sit there and watch the almost dead thing die last most times.
Because I'm not wasting my Life surge'd FT on a mob with sub 5% hp, start whacking that mob tanks.
Killing enemies is a DPS loss