Of course the Model is on the server, that is what MVC is for. My controller and my model is on the server. My View is in the browser. Some times however, my view is very, very complicated. Think something like a "wizard" interface with 5 steps or more and a plethora of paths through that "wizard". If I run back to the server constantly to get View information, in other words, what user input screen to show next, I have created an application that will not scale. It will also not be anywhere near as useful as a desktop application, since the user can not leave the app in the middle of the "wizard" and come back at an arbitrary point in the future to complete it. I am less concerned about #2 though. Scalability is my main concern, and complex client applications with server-side state simply do not scale. Don't even think about it.

In such cases I need to create an intelligent client app. One that can keep state. One that doesn't have to go back to the server for everything. Then I would shoot my self if I had to do it in JavaScript. CoffeScript improves the situation. GWT makes it a lot better. We're still nowhere near the ease of development you get with Flex and Silverlight though (who both have obvious drawbacks - and no, Microsoft killing SL is not one of them since they are not).