As a MNK I love having a SMN as the other DPS in dungeons (Expert roulette ones). Had some very fast clears that way. MNK+SMN power combo!

Double SMN would be really good too, I'm guessing.

In general, I don't think there's any DPS that -isn't- welcome, as long as it's played well. Double BRD/MCH in a dungeon will be a bit slower, but I had it multiple times levelling my MCH and it wasn't all that bad.