Originally Posted by
Shieldarm
As far as the adds, I look at it this way. They need to die regardless. If it's not preventable to stop their spawning (I've seen some pushes to mounting before an add wave was able to spawn), then they need to be killed. In this case, the only wasted damage is the dots and "fire and forget" aoes that don't deal their full damage. Instant damage, whether on the boss or the adds, will have the effect that was needed. Can some abilities have been better served as boss damage? Of course. But, it's basically the difference between a boss dying 5 seconds earlier, or the adds dying 5 seconds earlier.
Alternately, if the boss is NEAR death, then add damage is pretty much inconsequential. Unless their survival would prevent a boss kill, then a final amount of adds are generally able to be ignored, to prioritize the boss.
That said, we also can't forget, that Xuen has a 3 target cleave. So, it's not like he's useless on adds too. Just not as effective. While being more effective at boss damage.