I don't think it strengthens the community. If someone wants to make a change a library,
but not release the source, they cannot do that with GPL.
The idea behind GPL is that then, the change is forced to be released - which would, as you say, strengthen the community.
However, I think what would happen instead is that the person would simply not use the library in the first place.

So in short: GPL does not make people become a part of the community - it pushes them away.

Jonathan