But until those 18 months are over nobody knows exactly what that code been doing, and nobody could fix potential flaws of build on top of that idea, which I feel is a big part of what is so great about the open source world.
Innovation can now come from anywhere and be paid for. Someone might read the source for a web browser and come up with a great new compression scheme to lessen network traffic. This outsider would then code up his method, contact the hybrid-source vendor and license his source to them, to be sold as a closed-source patch on the software. Of course, his patch will also be open sourced after 18 months.