The GPL has been tested in court and the license holds up. I suspect many cases actually settle because they were not winnable for the violator and they were looking at substantial damages if the case came to trial. In this case there's usually some kind of confidentiality agreement [1].

If you ship GPL code and don't release sources for the entire derivative work that is a very shaky economic model.

[1] http://www.americanbar.org/publications/gp_solo/2012/novembe...