The Unobtrusive Validation and AJAX support in ASP.NET MVC follow many best practices that enable Progressive Enhancement and are also super easy to use. The unobtrusive AJAX library (not the unobtrusive validation library) is admittedly a bit limited in functionality, but if it satisfies the requirements of the application you are writing, then by all means use it. And because the source code of it is in your app (it's JavaScript, after all), it's generally straightforward to make any updates or changes to it as you see fit.
If someone wants to write everything manually, they are welcome to write assembly code to solve all their problems. Or just etch 1's and 0's onto a DVD with a laser pointer.