Well, it partially depends on what you want to do. We use ExtJS when we want a highly interactive UI, and that clearly lends itself to AJAX calls back to the server. Of course you also end up with some business logic in the front end unless you want to make hundreds of calls back to the server to validate data and what not. Of course then you need to validate on both end since it is very easy to send data to the server without the JS in the middle.
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