In my opinion (I'm a Java dev), the main benefits are being able to run RoR apps on TorqueBox[1] or some other JRuby server[2] and benefit the clustering, HA and other stuff that go with it; and 'the use of "true" native threads'[3].

[1] http://torquebox.org/ [2] https://github.com/jruby/jruby/wiki/Servers [3] http://www.restlessprogrammer.com/2013/02/multi-threading-in...