Sorry for the late reply - I was on vacation.
However, I think Charles got my intentions right - nevertheless here you have my two intentions with this feature:
1) To control the standard marshalling behavior of JRuby: As far as I know, there is no way to control in JRuby which class in Ruby should be used for a Java class - and the other way around. This is sometimes really annoying - especially when using Date/Time objects. There are plenty of DateTime implementations out there in Ruby, but I am restricted to use the Time Class, because this is the standard "definition" in JRuby.
2) To extend JRuby: I have implemented a Time span object in Ruby and in Java. They are basically the same and I would like to have a way to tell JRuby exactly about this relation - that I do not need to take care about this conversion by myself.
This manually conversion topic gets even more complex when working with Hashes/Arrays. I need to take care about the conversion of the key and values which is cumbersome and also robs a lot of performance.
(Btw - the topic with hashes is generally a little bit annoying because the Java method "get" and "put" of the RubyHash Java class does not call the equivalent method  of the Ruby Hash method which is an issue when working with default values of non-standard hash implementations. Nevertheless I understand that this has something to do witch caching and performance...).
Please do not get me wrong in any way. I really like what you are doing and I think JRuby is great - this is also the reason why I introduced JRuby as a core component to the product I am working on. But I guess that I am one of a few which are using JRuby that way. If you need any further information or clarifications, just hit me back (I am on a business trip until Friday).