Im but a low level monk, but in making the comparison between monk and dragoon I havent seen a few things mentioned. This is all theorycrafting, let me know if Im missing something.
1. Mnk has better party support through enhanced mantra and the INT reduction on dragon kick. Enhanced healing and reduced boss damage vs buffing one of the weakest dps? Mnk seems to play better with others.
2. Mnk has more damage multipliers over dragoon, leading to increased limit break damage. Most parties want to bring one melee dps for lb, so why not the one that can push more damage?