Q:

Is MVP always worthwhile?

Is MVP considered best practice for *all* GWT applications?  

What about for a small in-house web application?  
Is it a mistake not to use unit testing for such application, or any application?  
For an application that is not going to be unit tested, are there other reasons I might want to use MVP?

A:

In general, MVP is considered a Best Practice for developing GWT applications. Several reasons to apply the pattern can be found in this Google I/O Presentation.
MVP comes to a price, and it will introduce several more classes and complexity to your application. But it will also minimize coupling and it will make your application more testable; so it is up to you as a designer to decide if the pattern is worth the pain.
Some methodologies - like Extreme Programming - encourages the inclusion of automated tests in a software development process. Again, the inclusion of tests will require your team to write more code, but in exchange your code will be robust and  trustworthy. I strongly suggest you to include unit tests, even if the application is small.
As I said before, easy test is one benefit of MVP but it is not the only benefit. According to this article from Google: 

MVP decouples development in a way that allows multiple developers to
  work simultaneously

