Quote Originally Posted by YuriRamona View Post
If you're applying DoTs evenly across enemies and also spreading your single-target weaponskills, then almost all enemies will take longer than 18-30 seconds to fall.
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In this case you may be maximizing damage, but at the expense of other things. For example, when you kill a monster it can no longer deal damage to you, which means the healer and tank's jobs gets easier. However if you deal equal amounts of damage to all monsters you'll have to put up with the max amount of damage sustained for the entire duration of the pull.

There are also many situations where an enemy group isn't sitting in a tiny little compact space. You've got ranged monster hitting you from afar. To spread dots and single target weaponskills out evenly would require the melee dps to run from target to target after every GCD, which would ruin your dps.

There are also many situations where a single monster poses way more of a threat (or maybe just an annoyance) than other monsters (like the succubus in AK, who can hit you with powerful AoE magic) and should be killed so that threat is eliminated. This is also the case in dozens of boss fights. If you don't kill the adds, they'll eventually overwhelm you.