Right, websockets servers tend to need NIO otherwise you just wind up with too many threads to manage. However, streaming has uses outside of websockets; for instance, returning a response that's too large to want to hold in memory.
Or simply flushing out the <head> of a html document before the body is generated so the browser can start pre-fetching resources. That's the main use case that motivated Rails to implement streaming.
Of course non-blocking is what people want for stuff like websockets in most situations.