TLDR: Rails Live Streaming allows Rails to compete with Node.js in the streaming arena. Streaming requires application servers to support either multi-threaded or evented I/O. Most Ruby application servers are not up for the job. Phusion Passenger Enterprise 4.0 (a Ruby app server) is to become hybrid multi-processed, multi-threaded and evented. This allows seamless support for streaming, provides excellent backwards compatibility, and allows future support for more use cases than streaming alone.
Interesting post on Rails 4 and Rack streaming support. The way Rack implements streaming is a bit different from PSGI, but the analysis on multi-process (Unicorn, Starman, Starlet), multi-threaded (Puma, no equivalent in Perl PSGI) and evented (Thin, Twiggy, Feersum) is quite spot on.