Yeah, I also gave up on the Monk after carrying him half way through the game and he still remained obsolete.

However, Black Belt is another case. It took two dungeons for my Black Belt to catch up to my Knight. Overtime, the Black Belt reached my Knight's damage and he is now doing more damage than she is, and she has the two most powerful swords in the game. And no, my Knight doesn't suck and she's over job level 80. =P

The Boost ability is pretty worthless by the time you get it. I mean, at this late point of the game; If I was already doing over 7000 damage, and it sacrifices a turn to make me do 9999 damage, what's the point? D:

So in short, forget the Monk, have he/she play another damage job and switch to Black Belt later if you desire a good damage dealer.