A number of scenarios that are common in feature descriptors cause issues with the validate goal of the plug-in. Support is better in the trunk; however, at least one of the scenarios is still unsupported.
The fixes must be backported to the 2.2.x branch as the trunk does not validate feature descriptors without a namespace declaration and many existing feature descriptors that are transitively included in validation do not currently include the namespace declaration.
The scenarios in question are:
1) A blueprint file referenced from a bundle element.
2) Loading bundles from a repository other than the default repositories for PAX MVN/URL.
3) Loading a WAR from a bundle element.
4) Loading a plain JAR with the wrap protocol in a bundle element.