As someone who's written and released his own Free Software, I have to say, I at least definitely cared about getting users. Most of the fun is in your product actually being used, not just it being out there. I never did anything remotely as important as Gimp, but if someone had come along and offered some simple services like "Let me design a cool logo for your project" I'd have jumped at the chance. I can't imagine Gimp hasn't had similar opportunities.

If they want to charge consulting rates or if they want to get a job somewhere maintaining it full time, then adoption matters.

Gimp is challenged in the UX department as well. Getting a professional onboard would help with that a lot too.