Learning a framework is just like learning any language out there. You would have the same issue if you wanted to start to write an application with .NET, Rails, Java, Python, etc.
MooTools is a compact,
[Can be used in parts]
designed for the intermediate to advanced
It allows you to write powerful, flexible, and
[It deals with the browser quirks]
code with its elegant, well documented, and coherent API. [compared to other frameworks the api is nice]
You are not forced to use this thing [unless your boss says you are]. There are plenty of frameowrks out there from prototype to JQuery to Dojo and so on. There are 1000's of them. Find one you like.