The same reason why we don't pack all of ejb3 or all of jboss together: we want to be able to upgrade individual components.
The benefit might currently not be clear because we only have one implementation for each component. But in future I want more caching and pooling implementations.
Then "plugin" would depend on core and each of the projects' client classifier versions. Very possible for the client classifier versions to not match the project versions obtained transitively via core.