I don't say Sanji is weaker "because he's the cook," I say he's weaker because he can't rip a train in half or anything like that. And he doesn't intend to, either.
@Paulie:
But Luffy's not going to fight others OVER One Piece, seeing as how, y'know, nobody else can get to it anyway. Sanji's not going to have to fight to get to All Blue, either. But I can guarantee BOTH OF THEM are going to be fighting the wicked they meet along the way. They're both going to fight Marines because they're both pirates, regardless of their dreams. They're both going to fight to protect their nakama.
So how does Luffy really differ from Sanji in this regard?
Just because he's the cook and his dream is finding All Blue doesn't mean he's going to be taking a backseat role when the fights roll around. He's not going to say, "Aw, hell, I'm the cook, not the captain or swordsman!" He's going to be right up there with Luffy and Zoro (and now Usopp) fighting whatever enemies they're facing, not caring how strong they are.
So I think anybody saying "So and so has to be stronger because of their role" or whatever is vastly mistaken.
But you see, Luffy's goal is something many others are after, and at least one is insanely strong. The chance of Luffy finding it, taking it, and having it without major fights coming his way are close to impossible. I'm pretty certain he'll have a series-end battle.
Sanji's goal has nothing to do with being a criminal. And until this is settled, he's not exactly on the government's radar.
And even though they're all pratically adults, Luffy, as captain, still has a responsibility to watch over them. They end up in these fights because of him, as his supporters, and the biggest fights always come his way.