TestNG's native XML output is way richer than JUnit's output. It specifies details about which suites were invoked, which classes contain which methods, which parameters were used in parameterized tests, etc. It can also include the messages printed to stdout, as well as logging information generated with Reporter.log. It's also the basis for TestNG's "rerun failed tests" feature.
Several other bugs and enhancements can't really be done until we do this.