Weapon Usage

Then uh... how can you forward roll with them? I can hardly imagine that...

It's a fighter skill; That should be reason enough. A lot of things in Mabi don't make sense, so there's no point really having this discussion just because you don't see how it can work in a game.

Yeah, and you can use most Combat Skills and most Magic Skills with any weapon?

Magic and melee were around since beta, when things were more simple. You just pointed out exceptions. Now apply that to tumble and you just answered your own question. Does it make sense that you need a shield to charge? (excluding giants and reforges from this) Nope, but in real life, you could be butt-naked and charge at someone. Does it make sense that you can activate chain cylinder without having a cylinder equipped? Does it make sense that giants can't use bows/make bows that they can use? Again, the number of things that don't make sense in Mabi makes it a bit pointless to really question it.