What makes monstrosity fun in FFXI :
- Customisation. but if the devs of FFXIV have one day the good idea to add customisation somewhere in the game, they better add it to "human" character first.
- Personality. In FFXI, each monster type have distinct ability/traits that actually play a role in their dangerousness. In FFXIV, trash mobs are trash mobs.

Thus, if they were to add monstrosity in FFXIV now, it would just be for appearances.
(like everything else in the game, indeed...)