Come to think of it, is there ever a good justification for using GWT?
In fact, the only kind of library I could consciously condone is one that promotes object oriented design by not giving you so many easy (but SLOW) ways to access DOM elements (I'm looking at you, $()).
Of course, it all depends on what you're trying to do. GWT tends to ease the data transfer a bit, from what I know, plus being easier (I assume) to integrate with Gears. So if you're doing a lot of data manipulation and storage with AJAX, GWT may actually be the way to go.
If you feel comfortable writing JavaScript, that's probably the direction you want to take. GWT seems like a platform for Swing programmers who don't want to deal with JavaScript's idiosyncrasies and browser quirks.
But for all I know, I could just be blowing smoke - I've not delved that far into GWT. Seemed like too much trouble for what you got from it...
You say: "GWT seems like a platform for Swing programmers who don't want to deal with JavaScript's idiosyncrasies and browser quirks." But couldn't you say the same about jQuery?
In fact, GWT requires you to understand the DOM and to get into the more advanced tricks with GWT there are features to let you write js directly in your Java code.
Not sure what you mean by "ease data transfer": "If you're doing a lot of data manipulation and storage with AJAX, GWT may actually be the way to go". Do you mean if your ajax passes a lot of data back and forth, then GWT might help because its minified and optimized?