What's that have to do with anything? Robin has a bounty too. Anyways, people don't even notice Sanji's gone, both allies and enemies. He'll just vanish for a few chapters and then come back at the last moment with I JUST FUCKED UP YOUR PLAN, BITCH
But to the point, I was more thinking of someone who's entire purpose is spying. Like Bon Clay, they actively disguise themselves and sneak into the enemy's crew, or the Marine bases all the time.
Plus, they'd be the perfect person to fight that guy from Blackbeard's crew? Lafitte, I think?