The question was obvious to me. The advantage of client-side code is responsiveness. Everything is loaded, there is no need to go back to the server. Better yet, you can create UIs that couldn't be created otherwise. For instance consider gmail's keyboard interface. Just hitting keys you can navigate through email, delete some, label others, choose to reply to more. Without hitting a mouse. It is a wonderful UI, but only is possible with client-side code. Which is likely to be saner if you have a good client-side MVC model.