How well do free-to-open-source-projects policies work in practice?

Question

In comparison with an open source license and requesting donations, is a free-for-open-source-projects (or free for non-commercial developers) closed source and otherwise commercial project likely to get more license fees? Or just to alienate potential users?

Assume the project has value to programmers - I'm looking for generalizations here, though specific examples comparing existing projects will be very interesting.

What I have in mind involves code generating programming utilities. And one issue I can think of, either way, is a near total inability to enforce any license restrictions. After all, I can't go around the internet demanding that everyone show me their source code just in case!