What are Groovlets good for
My Groovlet experience
The first time I started using Groovlets I saw them strictly as a prototyping tool. My client was having some trouble imagining the flow through the partially constructed site I was working on. The team used Groovlets to fill in some of the missing pages and mock up some of the UI elements that would be present in the final site.
Then we started using them to expose what was happening in the database, including fields that maybe we were not going to expose on the website but mattered from a backend perspective.
I always thought that ultimately having dynamically evaluated pages would not really scale and performance would dictate when we removed the placeholder code but it turned out that for the project there were no performance issues with using Groovlets and in fact ultimately we benefited more from the productivity boost using Groovlets gave us.
So we ended up styling our prototype pages nicely and simply declared "Mission Accomplished" the client had seen the pages and been using them throughout the project. There didn't seem any advantage to reimplementing them in Java and Freemarker.
Towards the end of the project I was starting to write all my Ajax data providing scripts in Groovlets and I was starting to wonder what it was that Groovlets weren't good for.
Of course they looked strange and broke a lot of the MVC rules but they were providing me with rapid development with no obvious downside.