Even though the quote was actually from Shaw, Kennedy's quote is highly appropriate in Kinsella's work. The basis of the narrative is that Ray sees things that others do not. When people like Mark question Ray about why he does what he does, Ray responds on a different level. When others do not see the baseball game being played on his "field of dreams," Ray does. When others cannot understand what the messages mean, Ray does.
The collision between the world's inability to embrace dreams and Ray's incessant need to see reality as it should be is where the quote is highly appropriate. The context of the quote is a fundamental crash between how the world is seen and how it can or should be seen. This is how Ray stands in opposition to nearly everyone and everything around him. Ray's continual drive in the novel is to say "Why not" when no one else does. This animating spirit of dreaming is what motivates his character. It is also the fundamental spirit of the quote, making it appropriate to use in Kinsella's novel.