Brook is a musician. His job is playing music and providing entertainment to a crew that already has the most entertaining captain. I don't think it's narrow minded to think this. All of the crew members usefulness is becoming increasingly obvious, especially in this arc - Zoro's leadership, Robin's archaeology, Franky's building and craftsman skills, Usopp being the sniper and the only one capable of helping beat Sugar, the absence of Sanji resulted in Luffy almost dying of being poisoned because they don't have a dedicated cook anymore, Nami's navigation skills helped them reach Big Mom's territory, Chopper kept Luffy alive when he was on the brink of the death.
And then there's Brook.
When he first joined I thought he was great but now he feels like an odd man out. He almost feels forced and as if he has zero purpose.