Synchronous messaging is insufficient for making multiple requests at once and waiting for the responses. Also asynchronous messaging allows for optimistic notifications -- for example to clear a cache when coherency isn't required.
With asynchronous messaging it's simple to build synchronous interactions if desired --- send a message and wait for a response; but you can't build asynchronous interactions on top of synchronous messages.
Let's say you are writing a parallel crypto hasher... Which if you're part of a pool is the case at the nacro level, the first to get to the answer should probably cancel the rest (though strictly speaking it's not necessary)
