Why? You're not losing anything. It's rather the opposite, you can only win. The more people can work with the code, the better.
I disclose every source code I have ever written and I never had any bad experiences. I even had people pick up the code and send me patches which dramatically boosted development.
I do that on occasion, and people still send me bug reports rather than patches.
I've actually had more success with closed source projects - people ask for the source code, I provide it, and stuff actually gets done.
Having said that, it does annoy me when I find something on Aminet that I want to update, there's no source code and the developer is long gone.