It would be really nice if someone from Splunk answered this question. The documentation for Splunk 6 includes an entire manual on advanced xml yet I see this question/answer which has my head scratching whether to continue to use advanced XML: http://answers.splunk.com/answers/109408/splunk-6-converting-simple-xml-to-advanced-xml-with-saved-s...
"The use of Advanced XML to customize is no longer encouraged."
The impression I've gotten is that Advanced XML and the Modules API it uses is being replaced by the Web Framework, but the docs don't really reflect that. It's confusing to have 3 different languages for dashboards, though.
I agree a response from Splunk would be good!
Advanced XML is a legacy technology, it is true. The focus now is on simple XML and the Web Framework. But it is not a deprecated feature at this point.