Now to be fair, a lot of actor systems were about correctness and not squeezing the most performance of a platform. Not sharing data means no explicit locking with mutexes (though one can still have deadlocks, as in several actors stuck waiting for each others) and something simpler to analyze: the system is made of communicating state machines. Actors were also used in high performance system like telecom switches (high performance, for their time), but here too correctness was probably the main concern. At the time actors were first used accessing even far memory was not as costly as today, so the cost of synchronization wasn't as bad. But it was already tricky to get right.

Still, actors are perfectly on topic when discussing a shared nothing, message based architecture. They're worth mentioning IMHO, to give some historical perspective. And then one can combine both approaches: a pinned thread per core can dispatch its messages to a set of cooperating actors for example.