I think the idea of message passing is a lot like "pure" functional programming. What matters isn't simply that the language does message passing but that the programmer can count on all communication being by message passing.

Then you have a broad "polymorphism" that guarantees all the parts of the system just take messages and do things. Obviously, in practice that could be implemented by function call but having the confidence that programming units are only going to receive messages makes it easier to think about the system - hopefully.