Some things MNK brings to the table:

- Traited Mantra
- Fist of Earth
- best melee AOE
- DK (assuming no DRK)

So, basically, MNK's main advantage is bringing defensive options and great AOE. The reason you'll often see people say MNK's not optimal is that if you don't need those defensive options then bringing a DRG in it's place is better gDPS (due to Litany and +BRD/MCH DPS).

However, if you don't plan on raiding hardcore world first then the DPS difference between DRG and MNK shouldn't matter. Every job in the game is viable for raiding, the only place where balance is absolutely key is in world first early prog because you'll want every bit of an advantage you can get. So if you enjoy MNK, play MNK.

Also, just as a side note, DK is also useful in the DRK/WAR comp - as an easy example in A1S you can DK the WAR's boss while DRK applies Delirium to the other. It's pretty niche, but it has it's places.