Except the GPL doesn't just encourage open sourcing works which use the GPL work as a dependency; it makes the open sourcing of any library/program/what-have-you a MANDATORY condition for use of the GPL work. This has nothing to due with modification of the GPL work; the condition is still in effect when no modifications, proprietary or otherwise, are required in the GPL work.

I'm actually in agreement with Ballmer on this one: GPL is cancer. And I love contributing to OSS in general.