Just a point of interest, I was in a group with 2 dirges, a troubador, an illusionist, a berserker, and a fury healer. Levels 52-60.
We blew through "Scornfeather's Roost" with several member's commenting that it was the fastest they'd ever done it. (a relatively high level instanced zone...good loot, several named fights).
My point would be, there was enough diversity and available buff choices that this group was not only viable, but very efficient.
I have a lot of problems with EQII, but their ability to balance and add flexibility certainly isn't one of them. A dirge can go offensive on his buffs or defensive...with two dirges you get both. The troubadors buffs (except power/mana regen) all stack with the dirge. The illusionionst's power regen stacks with the bards.
During peak moments, haste was 110%...probably the cap for EQII. Non of those classes are in the '1' category for dps, but when you are buffing up 4 toons with your songs, it more than makes up for it.
It wouldn't have been as viable with 3 dirges, i don't think, but we still could all have chosen a different base concentration point buff line up that would have had only only one duplicate on 2 of the dirges.
This was a fun group, and instanced zones on my server (the only Merged server so far) aren't over laggy (for me).