Out of all the terrible and sad things that's happened this arc, Brook mentioning that he's left playing the accompaniment hits me the hardest. To play the accompaniment by itself lends a song to a very specific feeling of incompleteness. It's always supposed to be in the background, not noticed compared to the main melodies and harmonies, which represents Brook's crew as much as it does Bink's Sake itself.
Togetherness and camaraderie are integral parts of the series, and it's interesting to see how Oda plays with isolation and loneliness in different ways with different characters. Unlike Chopper or Robin, Brook was never rejected by his peers--they were ripped away from him by circumstances beyond his control. And because of that he's left playing one part of a song that he's used to being performed by an orchestra of his dearest friends. That adds a welcome dimension to a theme we've seen explored throughout the entire series.
Brook is eighty-eight years old; he has an entire lifetime of experience and adventure behind him. Despite that, he doesn't sink into the mires of his past, and instead jumps at the chance at new adventures and new friends, knowing that there's a risk that they, too, might die. That takes a very specific kind of courage that the older I get find myself appreciating more and more.