At least with a dominant WebKit, everyone has access to the source code, and can either submit their improvements back to the core maintainers, or they can fork

At least with a dominant WebKit, everyone has access to the source code, and can either submit their improvements back to the core maintainers, or they can fork WebKit if the maintainers refuse to allow their improvements. With a dominant IE, you're at the mercy of some "unknown" Microsoft engineers/managers finding enough time in their schedule to implement those improvements.