We used to have a DRG in our group but he ended up switching to MNK.

Thing is you have to look at the available slots. You need 2 tanks in final coil cause Ugghhh tank swap mechanics made their triumphant return... 2 healers is a given, you need a BLM for certain Magic damage mechanics and SMN doesn't cut it, a bard is great for foe since you have that blm, paeon and ballad also. So that leaves 2 slots. MNK offers dragon kick which helps reduce damage from certain magic attacks so more likely to bring a mnk than a drg, so that leaves one spot, which is more likely to go to NIN than drg, since although drg offers the BRD a 10% piercing buff, the NIN offers the Mnk goad which means the BRD won't have to paeon as often. NIN has less positional requirements than drg and also can offer some magic damage with ninjutsu to help with those magic mechanics.