One major goal of Qt 5 is to make JS and QML a first class citizen in Qt. The best way to ensure this is to move the support for it (QJSEngine and QDeclarativeEngine) into QtCore. This also allows us to use certain pieces of V8 (regexp, dtoa) in Qt, reducing the overall size of the stack and giving 100% compatibility between C++ and JS.
The move can't happen just yet, as the integration code still needs to stabilize a bit more. A possible first step would be to move only V8 into QtCore (or into it's own shared lib that QtCore links against.