Benefits of shpping the source code:
– Consistance. If I have the source code for the JDK, why do I have the Struts source code?
– Easy to use. Everyone knows that Struts is open source, but may be (just may be) not everyone knows the best way to associated source files with a jar file in Eclipse project.
– Convenience. If I have 20 web projects I do not have to set up the source code and the javadoc for each one of them.
I believe that what IDE stands for.
And by the way, the source code for struts is about 1 MB zipped.