The other thing about async is that, in some scenarios, it can make shared resource use clearer - i.e. in a program I've written, the design is such that one type on one thread (a producer) owns the data and passes it to consumers directly, rather than trying to deal with lock-free algorithms and mutexes for sharing the data and suchlike. A multi-threaded ring buffer is much less clearly correct than a single-threaded one.
