The other point is that the more extra DD's you add on to a small man dungeon, the less each of them individually matter. You've got one healer: if he screws up, everyone dies. You've got one tank: if he screws up, most likely everyone dies (unless you can go kite just fine). You've got one DD: if he screws up... you just kill it a bit slower? (obviously the exception being bosses where you have a DPS check). I just feel like for every DD you add to your small group, the less each individually matters (unlike having an OT which makes adds significantly easier and you have a backup plan, similarly with a 2nd healer if one healer dies the other can raise him, also you have benefits from the SCH shields WHM strong heals, one doing the healing while the other damage deals, etc.).

The more DD's (or DPS's I guess if you came from a non FF MMO), you just in the end get more damage. Maybe more utility from silences and such.