TBH any weapon style is a “martial” art. I understand the history of oriental styles vs occidental; however, to master a weapon still requires a skill set that takes years to learn.
For example, what is more effective? A short sword and shield combo vs a great sword? Depends on the training and situation. If sword and board was so effective great sword style would have died out long ago. You just don’t use a great sword by just swinging it. The blade is set higher so it can be used like a Jo Staff, the hilt done in such away to allow it to be used like a spear,you can use brute force to bash, or you can swing. So many options. However as shock tactics began to become more organized (think Roman legions, pole arms, etc) the great sword began to lose some if it’s functional aspects.
We aren’t dealing with shock troops or infantry lines. Each battle is more one on one, so the skill in a weapon can be paramount.
Occidental weapons styles may not be as elegant as oriental, but the basic martial skills are there. That’s why I don’t see any issues with a monk using different weapons. I would prefer the old world weapons (Jo Staff, Nan Chaku, Kusarigama, etc), but I am fine as it is (especially multiclass).
