I was a previous user of the XMLBuddy plugin for Eclipse.
I'm happier with Oxygen, but I miss this functionality that was very useful albeit simple:
the possibility to associate automatically a schemma, DTD or relax NG to an XML file, depending on its root element or declared namespaces.
For example if <html> is the root tag, I will assign the XHTML DTD.
I have lots of files, and create new files often and it's very cumbersome to assign to each one a schema manually.
Having trouble installing <oXygen/>? Got a bug to report? Post it all here.