Arfrever: I doubt anybody has contributed patches back, or that anybody is interested in doing so.
I personally don't see a problem in using an old libffi version, so I fail to see Benjamin's issue. Figuring out how exactly to use the system libffi is more hassle than keeping our own copy.
Please understand that ctypes is unmaintained. Anybody actively taking over maintenance of ctypes would have to decide on how integration with libffi is supposed to work. Without a maintainer, falling back to the sytem libffi is a too high risk, IMO, since this will certainly produce tons of new bug reports, with nobody prepared to deal with them.