Cognitive Load, Portability and the Superiority of Client-Side Frameworks
The other big advantage to developing UI code in its native language is that you can port it to any server platform. With GWT and similar frameworks, you've got to rebuild much of your UI from scratch if you want to change course in mid-stream. With purely client-side frameworks such as Prototype, jQuery, YUI or MooTools, switching libraries may entail rewiring some of your code to a new API. But switching server platforms, from J2EE to .Net to PHP to RoR, can be done without much work at the UI layer. "The right tool for the right job" is a truism for a reason. Pure client-side development of UI code allows for the development of reusable components whose only dependency is on the standards bodies and browser vendors who control JS, CSS and HTML. GWT and its peers are useful for some teams and some projects, but they're not the only answer to webapp development.