There is a second part of the patent quid pro quo deal that is often glossed over and that is "... without undue experimentation". For many ideas, the idea is enough to allow someone to reproduce it, but for a large subset of those ideas, a person skilled in the art would still have to go through undue experimentation if there is no source code available.

Source code is often the only way for someone to recreate an idea disclosed via software patent without undue experimentation.