[gillianseed]

>Doesn't GPL effectively prohibit combining it with a closed source commercial product?

Well yes, since if you sell it to someone you need to offer them the source code should they want it.

However the incentive here (as I understand it) was to allow MAME to be used in museums and the like where they charge an entry fee (thus commercial use), and not in order to make it easy for people to create closed source commercial forks of MAME.