The Doctrine metadata stuff is very coupled as I remember, and seems to be reinvented often IIRC (e.g. DoctrineExtensions). The JMS stuff provides a good base and saves alot of headaches :)

Regarding making it a suggestion, its worth noting the possiblity to make other Routing stuff mappable, e.g. route controllers / templates. So the metadata stuff could be potenially be more in the "core" of the bundle.