To explain the need: The obvious use case is of course the JBoss service one, where the lifecycle methods are used for pretty much every MBean.
Addtionally, users may want to enhance existing services. Whereas with standard MBeans you simply extend the MBean interface, there's no equivalent for XMBeans.
There should be a schema based version that allows for inheritance of attribute groups and the like to allow for this. Having to step outside of the xml features for this just does not seem worth it. We do have entities in the dtd for resuing commons ops/attributes. This is what would have to be used for the existing xml/dtd based version.