There is an important difference between functionality and usability.
On a webpage, Functionality is the ability to do something. For example, submitting data on a form, performing a search, or even clicking on a link and accessing/viewing a web-page. All of that is functionality, all of it needs to be accessable by every user on your webpage. This is also known as accessibility, and is an important part of design.
Usability is how something is done, rather then what is done. So to enhance usability for something, it works on making the functionality easier to use (in whatever form), without adding extra functionality. This means that if the usability enhancements are not valid for a specific group of people, the functionality is still accessable. A good example of this in the world is the humble handrail on a flight of stairs. It is there to enhance the usability of the stairs for those who need something to hold on to, while others might for whatever reason (hands full for example) won’t use the handrail.
In the realm of websites, usability is important and if a pages usability can be enhanced by client-side scripting then it is welcomed by most users. However for a user without the required scripting language turned on, the page still functions the same they just lose some of the usability.
So, the moral of this story? Write JUE on your website, not AJAX!
Note: It is important to note that when it comes to using JUE (or AJAX), it is how it is used. You can call it whatever you like, but if you miss-use it, then not even a fancy well-intended name will save you from its lies.