[Mentathiel]

I think what you need to consider is that, at least while levelling, a group needs one cleric and one tank. Warrior and cleric might be 'god tier' in Susano's list and they are the essential components that a group is built around, but you rarely see two of them unless you are in a specific location; two clerics in a Befallen / Unrest / Mistmoore group is not uncommon for example, but mainly because of the undead.

Rogues 'stack' nicely. They never get in each other's way, they never compete for aggro and they never reduce each other's damage unless they do something incredibly stupid like hit the Instill Doubt skill by accident. Three rogues are three times as good as one rogue in most cases.

Also, until you hit the raid scene, a Paladin or SK is just as welcome in most groups as a warrior. I say most groups; some people are still scared of the exp penalty for some reason. Yes, a warrior has a 10% bonus instead of a 40% penalty, but they also cannot LoH to keep themselves alive on a bad pull or FD to dump the mob onto the off-tank.