GWT and Spring MVC, is it worth it?

I don't see any generally good point in using Spring MVC or another enterprise Java MVC library (like Struts) together with a layer which - as you said - offers only business logic (and therefore can be kept as small and clean as possible).

But Spring itself is way more than just a web (MVC) framework layer and using the dependency injection or AOP features or the ORM API or the Scripting language Groovy (which works fine with Spring) can be a huge benefit for any application.

Spring is much more than just MVC.

Even when you do your UI with GWT, you still need some kind of backend logic.Things like databases, transactions, security, additional services integrations (emails? SOAP?) and so on.

For this Spring or any other Java server side technology can be a good solution.

As daff said, Spring brings DI + AOP + transactions + many things...It is useful to have those stuffs managed on your server side with Spring.

Furthermore, the library gwtrpc-spring offers a very convenient way to declare POJOS as rpc services, with the @Service annotation. It avoids declaration of each rpc servlets in the web.xml, as the scan for classes with @Service is automatic.

GWT is just a toolkit, not a framework. If Spring can ease your dev, just use it.