The Pros and the Cons
Learning the basic underlying principles of web development concepts will always help to lay a solid foundation for any web developer to work upon. Otherwise the understanding of some of the concepts of web development may escape him. Learning about server side and client side concepts, cross browser scripting etc. will make the programmer less dependent on such libraries. For once one starts with one of them before learning the basics, they’re crippled.
Performance vs Power
Taking shortcuts without knowing what exactly one is cut out from the learning experience may actually be taking a long road instead of a shorter one. It is true that tools like jQuery and MooTools are powerful ones. But there will be a compromise on the performance for the power. Which web developer will really want to handicap himself, limiting his capabilities in the long run.