Yes, we'll drop the DPS by 20% constantly in a 4 man run because the other DPS has enough power to pick up our slack and keep the run progressing fast. *rolls eyes* In a 8-man dungeon, yes, that song should be going because you have 3 other DPS which can easily pick up the slack. But when there's just 1 other DPS, it's not worth the risk unless the WHM is running low on MP.
Judging by your class btw, it appears you're venting because you're a WAR who uses Flash. So here's my question...if you're complaining this much, are you also using Overpower, and tabbing between targets to single-target enmity boost each one in between Flashes? If you're not keeping the aggro up and you're complaining about lack of MP, it's most likely you being overly dependent on Flash.
