Why is a third-party class like TweenLite the best?

I would rather have a solution within Adobe's software package for various reasons, in a production environment, it's hard enough as it is for someone familiar with Flash and AS3 to sit down with an .FLA that they've never seen before, and written by an author who they've never worked with before... so, to introduce a third-party class just adds that much more of a headache. Also, it looks as though TweenLite has a "gs" folder that needs to live with the FLA... which presents another organizational problem.

ANYWAY, in the future when I'm doing personal or one-off projects, TweenLite may work, but in this situation, the BEST solution is to work with what Adobe gives you. So, anyone who can give me some input as to how to solve my problem within the constructs of the software would be greatly appreciated!