Metadata handling is useful when implementing AOEs, especially when coupled with an interception framework. It allows the weaver to extend the classes semantics in a non-invasive manner. Since most implementations on metadata allows dynamicity, it can also be used for dynamic configuration/reconfiguration of the aspects.
Even if the JDK1.5 will provide a standard implementation for metadata, it should be useful to provide a standard API that allows multiple implementations. These may take into account some environmental specificity such as distribution, serialization, that may not be correctly handled by the default implementation.