Not only would this make xforms mighty powerful by allowing custom
controls built from itself, it would also streamline the knowledge
necessary to build such xforms (people who know xforms wouldn't have
to learn javascript to implement XBL controls).
More importantly, I see this as currently the only option for making
xforms somewhat modular, and being such a verbose language, this would
be a major plus. You could keep your main document structure free of
clutter by relegating repetitive and verbose sections to an xbl
document.