I am still learning javascript and the more i learn the more i ask myself why do i need to learn a js framework like cappuccino, sproutcore, qooxdoo, smartclient etc...? The learning curve for these frameworks is steep, also wouldn't it be better to just make use of JavaScript libraries?
Take note, that my objective is to create desktop like web apps. Thanks in advance for any good view points!
IMHO, those JS frameworks you mentioned are overrated bloatware.
I would wholeheartedly recommend that you learn JQuery instead.
It is a much more minimallistic and elegant framework.
Also, by the time IE 9 finally ships earlier next year, all the browsers
will be supporting most of the HTML5 standard, including offline storage and
websockets, making such complex frameworks completely unnecesary.
Desktop apps are built with frameworks that are of the same quality as those from SproutCore, Cappuccino, etc.
If you want to build desktop quality apps with true MVC architecture you can either write your own framework on top of a library, or start with a framework someone else has written. (SproutCore was originally built on top of jQuery for example)
The best part of a true framework is that the build tools can include only the pieces needed to run your app resulting in very small code with rapid load times.
Seeing as you are still learning Javascript, I would suggest that you continue to work with Javascript, and try out tools like jQuery, and then depending on the individual application you are working on, evaluate whether you need to use a more full featured desktop style Javascript framework at that time - something that will require a knowledge of jQuery/Prototype/etc to evaluate.