I don't think that's actually true. 18 man Schah was recently cleared and I did an 18 man Warder of Courage. The DPS was really high, and on some fights even with DD's such as Erinys, it is better to have more DD. It really depends. There is a definite loss of adding even more than one melee DD in a lot of cases, SAM for example, and most every melee DD can self chain now. It's still ok to pair up as DD, but after that skill chaining goes out the window. And that's also ok on a lot of mobs, which have high magic resist and so that's why you're using melee anyway. And even if that isn't the case, some zerg strats do very well without waiting around for skillchains such as Might Strikes resolution warriors. There is a falloff, but adding more DD can still end up being more damage and at some point it can't really TP move much faster than it is. To be honest for some reason the people just after you add a 2nd party seems to make it a lot harder. But fully functioning alliances with two parties fully focused on dealing damage especially with a nuke setup in my experience if the people are well geared it's actually often easier than low man situations just because your ratio of damage dealers to support is so dang high. Those two geo and the SCH's making skillchains in rotations, are multiplying the damage of a lot of mages bursting, to the point it even overpowers the magic acc reductions if magic bursts are very close to each other.