I am still just learning AS3, but the first thing that got my attention was the "package" thing...

What is the point of that? You would never have more than one class in a single .as file... would you?

Is it really necessary?

moock's book goes into ins and outs but I just look at it as syntax rearrangement.

you can define private classes as well as 1 externally accessible public class in an .as file. packages organize classes into namespaces too.