I reiterate: That isn't a very pirate-y motivation for going to sea. Besides, the nobles themselves aren't really any kind of threat, just the people they're being protected by. It doesn't feel right to have a crew devoted to something like that and call ourselves pirates, especially if it's just a small crew that doesn't even comprise 10 members. Dragon is in charge of an entire army because of the nature of his intention, so at least he's going about it logically. Can't we just assume that the nobles were already wiped out during Luffy's lifetime leaving only the Marines as a threat to pirates on behalf of the nations of the world?