Not just source contribution, but also the contribution of new GPL applications. I've written a couple of apps which are licensed under the GPL, not because I necessarily thought it was the best license for me, but simply because I wanted to use a GPL library. Had the library I wanted to use been some other license, my app might have been some other license. But it wasn't and now there is one more GPL app.

Of course there is the other side of the coin where I haven't used a library I really wanted to use because I couldn't GPL my app.