Even though right now only the Google Eclipse ADT implements it, I think the AndroidProjectLibrary has some interesting tricks.

Think about this, you have some libraries that you want to create. One of the problems of a jar library use in android projects is that you cannot configure said library for example lets say you have a UI widget library and you want to use it a certain way and configured a certain way.  Using a jar you have to hand code the configuration each time you use the jar.

Whereas with an AndroidProjectLibrary you can set to read a file and serialize the objects in that file via the res/raw directory. Because the parent project will over-ride the same file in the res/raw directory you can put the stock configure file in the res/raw of the AndroidProjectLibrary and direct the developer using the APL to put their configure file for the library in the res/raw of the parent project. Neat, huh?