Being aware of the direct benefits of libraries having things wrapped in a package, making daily work easier, my question is leaning more towards the fact that many features not implemented in HTML/DOM/CSS, has become available using these libraries.
There is everything, from visual effects (13 a dozen) to more functional oriented, to choose from.
As I see it today, 2014, with HTML5 and CSS3, more and more of these libraries seems to have played there role, as a kind of a bridge covering the gap between older browsers non capabilites and the new ones ability to handle it built in.
Whats more, is that these libraries also seems to drop older browser support little by little.
This gives me as a developer a hard decision, if to upgrade to next version with more functionality and lose old browser support, or stay on an older version with less functionality but with old browser support and the need of custom development to achieve new functionality.