In RVM the `jruby` will always evaluate to latest version, today it is `1.7.3` tomorrow it might be `1.7.4`, other ruby environment managers will use only installed versions of `jruby` to pick one, putting the full version in `.ruby-version` create effort of updating it  every time new version of jruby is released - and produces a risk of sticking with older version because of not updating the file