I will always prefer a Monk + Dragoon over a Ninja for Melee composition in my groups, I find Dragoons & Monks to be more reliable than a Ninja (Longer time to get used to the class) and I usually just block off the Ninja slot on party finder because the Dragoon buff to Bard damage, and Dragoon + Monk melee damage is fine and not harming anyone in my groups.
This is from a Tank POV btw.