This totally depends on the type of application or feature you're developing. If it is crucial that the application is accessible despite the absence of Javascript, then it would help to have fallback methods (i.e. alternative forms) to allow your user to use said functionality/feature. For that, it will require you to invest some of your time developing methods for collecting information not just using client-side scripts but also on the server-side.
For miscellaneous features that only serves to enhance user experience, it's mostly not worth it to develop fallback methods.
There's no reason to totally not use AJAX. AJAX helps minimize your traffic after all.
