There is no further development on old libusb any more and the libusbx sources will replace libusb entirely (libusbx is the interim name only). Several Linux distributions already use libusbx named as "libusb", so I think it would be worth to fix this in homebrew and drop libusbx there.

In general it is currently not mentioned, that the libusb.gem brings it's own version of libusb and that it is used as fallback, if the system libusb is not usable. I would be happy to get this description from a native speaker :-)