> ...you should be aware of the implications of your choice.
> ...work may become much more difficult.

Indeed, but working with an inadequate schema language is not easy.

This is rather like deciding to use Ruby for web appliactions - it's
hard to find people who know it and it takes a little while to get it
fully working on your box, but once all that is done coding proceeds at
a faster clip and the code produced is better.

It may be the case that working with a less popular standard leads to
more work in the beginning - but if it's the standard that fits your
domain, than there's apt to be a lot less work in the maintenance
phase, when impedance mismatch is the principal cost. Work with less
widely used standards is more *front-loaded*, not necessarily more
costly overall. If it was, the business world would just stick with
inadequate standards forever.

I take issue with the suggestion that Relax NG is as a non-standard -
it certainly has OASIS behind  it. It may not be widely used, but
that's not the same thing as, say, YAML.