HE MUST!! Luffy said that he would drag Brooke onboard the ship.
And the pattern is their.
1. Luffy asks.
2. Said Person aoutomatically refuses.
3. The crew meets said person again.
4. Giant ordeal.
5. Said person helps.
6. You see said person's fighting ability.
7. Luffy decides that said person will ABSOLUTELY join the crew.
8. Luffy solves ordeal.
9.Person joins crew.
I mean, the patterns their. And everyone knows its going to happen.