Depending on the license, mingling open source code with proprietary is generally not wise. Unless your developers are very clear about drawing a line in the sand between proprietary and non-proprietary (eg no you can't cut and paste from here to there), you can get into trouble.

Sure the open source community is nice about it. Much nicer than the average corporation who you might have cut a deal with for code access. The FSF likes to get you to open source some affected code and then set up a voluntary compliance program rather than a lawsuit. But it is a real cost, and there is the fear that in 5-10 years someone who thinks that the GPL needs to be tested in court will be a real jerk about it. And depending on what your company does, you really might not want to unexpectedly have to GPL your code.

This doesn't mean that your approach is wrong. But the thought of developers who think this stuff is all free (and don't appreciate the legal risks) is what keeps corporate lawyers awake.