I've mentioned in the past why I think XML 1.1 was a bad idea in my post XML 1.1: The W3C Gets It Wrong. It seems at least one W3C working group, the XML Protocols working group to be exact, has now realized why XML 1.1 is a bad idea a few months later. Mark Nottingham recently posted a message to the W3C Technical Architecture Group's mailing list entitled Deployment and Support of XML 1.1 where he writes

In the Working Group's view, this highlights a growing misalignment in
the XML architecture. Until the advent of XML 1.1, XML 1.0 was a single
point of constraint in the XML stack, with all of the benefits (e.g.,
interoperability, simplicity) that implies. Because XML 1.1 has
introduced variability where before there was only one choice, other
standards now need to explicitly identify what versions of XML they are
compatible with. This may lead to a chicken-and-egg problem; until
there is a complete stack of XML 1.1-capable standards available, it is
problematic to use it.

Furthermore, XML-based applications will likewise need to identify
their capabilities and constraints; unfortunately, there is no
consistent way to do this in the Web architecture (e.g., RFC3023 does
not provide a means of specifying XML versions in media types).

As I mentioned in my previous post about the topic, XML 1.1 hurts the interoperability story of XML which is one of the major reasons of using it in the first place. Unfortunately, the cat is already out of the bag, all we can do now is try to contain or avoid it without getting our eyes clawed out. I tend to agree with my coworker Michael Rys, the day XML 1.1 became a W3C recommendation was a day of mourning.