The biggest problem with open-sourcing commercial software is the risk of a blatant ripoff.

Some entrepreneurial spirit will come along, replace the application icon, repackage the whole thing and start selling it as his own. Perhaps just in his own country, perhaps you won't even notice, but it will eventually surface and you will need to deal with the situation. Now the question is if you want to spend your time and nerves on resolving the issue or would you rather continue working on the product. Especially if you are a "lone programmer" and especially if opening the source doesn't give you much to begin with.