I use and like response documents because I like NotesDocument.getResponses() rather than rely on yet another view.
To answer your need: I think you'll need to instead use currentDocument.getDocument().getParentDocumentUNID()
currentDocument isn't a NotesDocument object, it's a NotesXspDocument object which is an xpages-specific wrapper around the NotesDocument. I don't know what currentDocument.getParentId() returns but I expect it refers to either the collection of NotesXSPDocument objects used on the page or whatever is above it in the XPages object model.