Well, Alabaster sees that Arthur is a good story-teller, thus having a good way with words. For that, a quick mind is required. So this would make him a helpful addition when it comes to decision-making in case Alabaster can't quite handle making one.
So to be sure that Arthur doesn't only have skills in thinking, but in battling as well. Thus he sends Takko (his other crew member) to have a battle with Arthur. However the battle turns out, Alabaster sees potential in Arthur and comes clean amd asks him if he wants to join his crew. That's what I can currently think of but...