That reminds me of Microsoft givng their source code to the European Union (or somebody, I forgot), supposedly so that the European hackers could check that it is safe.
Frankly, I don't think it works that way. People don't check a billion lines sourcecode for security holes just for fun. With normal Open Source software, the code gets some scrutiny because man different people look at it, and they are motivated to do so because they want to improve on it or learn from it. Both motivational factors are missing for commercial software, no matter if the source is open or not.
Even if the software is commercial as long as it is open source, you have motivations to tinker with it, to adapt it to your own problems and preferences and people will still look at it to learn.
Why is the fact that would the fact that you paid for this source-code make you not want to change that color there, that shortcut here etc...
Personally I wouldn't. If I tinker with open source software, at least I can publish my results, so it is that much more efficient than tinkering with closed source software.
Frankly, I don't think it works that way. People don't check a billion lines sourcecode for security holes just for fun. With normal Open Source software, the code gets some scrutiny because man different people look at it, and they are motivated to do so because they want to improve on it or learn from it. Both motivational factors are missing for commercial software, no matter if the source is open or not.