"But if you develop a point-of-sale system (say) and sell it to a store chain - they might not have any motivation to publish the source - even if it includes a

"But if you develop a point-of-sale system (say) and sell it to a store chain - they might not have any motivation to publish the source - even if it includes a patched mysql, or some gpl code etc (maybe a patched Linux kernel?)." They also have no obligation to publish it. You are distributing the software to them, you have to give them the source code along with it. They are using the software, not distributing it further, therefore GPL does not require them to publish the source code. (I'm not trying to argue with you here, as I think you know this - I just thing your comment does not make this detail clear enough. :) )