I know this is an old debate but I think it shows very nicely the different philosophies at place. If IDE is mentioned the answer should not be that it integrates nicely with vim. There is a huge amount of things going on in the Java-World that could be utilized if GWT was adopted. The YUI community is much smaller and developer time comes into place.
Much more important is the question whether moodle should become a more industrial style project with abstract modules, difficult frameworks and a much more complicated code basis or whether it should remain an easy playing ground for people to use their basic knowledge of javascript.
I agree with the conclusion that plugin developers could use GWT and there should be an affort to create something like a moodle-plugin-GWT-project to facilitate afforts in this direction.