An X509Data element is a declaration within a KeyInfo element. Thus the minor change of putting multiple certificates from a bag in an X509Data rather than one certificate each in multiple X509Data's. All the groups involved in current interoperability testing seem to think this minor change is fine. I'm sorry if you guys don't think the above is sufficient motivation for this minor change.
Most people seem to think it is.
And since you seem to agree that it is not a big deal, unless some consensus materializes to change it again, it will stay multiple certificates per X509Data element in this document.