It's a good question, but allow me to twist it a bit: what if your project uses
Struts 1.x and you decide to switch to Tapestry. There is two ways that you could look at this:
1. We are staffed with sharp Java developers. Even though the keystrokes are going to change, with a little bit of training they should be able to make the transition without a problem.
2. People here are afraid of change. They learn keystroke instead of concepts. Even the upgrade from Struts 1.x to 2.x would throw them for a loop. It's best that we continue to do things the way we always have.
Here's the rub -- with Struts 1.x, you are dealing with a page-centric MVC framework. With Tapestry, you are dealing with a component-oriented framework. I'd argue that this is closer to the Java/Groovy example. Java is statically typed, and Groovy is a dynamic language. But here's the neat part: you can use Groovy as if it was statically typed. So you truly get the best of both worlds.
Adding Groovy to your project is literally adding a single JAR to your classpath. IDEA IntelliJ, Eclipse, and NetBeans all have Groovy plugins that make the transition no-brainer for Java developers used to autocompletion, debugging, and refactoring support.
But adding any new technology to a project, be it Groovy, Hibernate, Spring, or anything else will be a "maintenance nightmare" if someone has never worked with it before and doesn't want to learn new things. They'll have to learn Hibernate instead of iBatis, Spring instead of Guice, etc. The good news is that Groovy syntax can be so much like Java that most folks wouldn't bat an eye.