But isn't the same true for the rest of the cast, with the exception of Ashe? Balthier doesn't have any great stake in the plot. The only thing he provides is an airship. Basch hasn't done anything noteworthy yet beyond being related to Gabranth. I needn't reiterate Fran's general uselessness. And Penelo is there as Vaan's conscience, basically. So, individually, none of the characters, except Ashe, would make a giant dent in the plot if they were suddenly removed. It's an ensemble cast. The whole of it is worth more than the sum of it's parts.

However, it can be argued that if it hadn't been for Vaan's brash behavior (attempting thievery in the palace), none of the characters would have congregated into the central party at all. Vaan tied himself to Balthier and Fran, which gave way to rescuing Basch, which gave way to saving Ashe. Without that first incident of theft, Basch would still be rotting in jail and Ashe would most certainly be dead. So, he holds more importance than what you're giving him credit for.