This is a dramatic change in ruby/rails infrastructure, not sure it is can be mature enough and being adopted widely. After all, many code is not friendly with non-blocking mode. It is going to be disastrous if ruby blocked somewhere for some reason.
It could save on memory, but I am not sure it going to be really faster than profork mode, comparing one non-blocking ruby handling 10 concurrent requests to 10 prefork ruby handling 10 concurrent requests. The non-blocking ruby may not able to take advantage SMP server if only one instance is used.
We will see how it evolves then make decision on whether to provide a non-blocking LSAPI interface.