XML catalog registration also for SGML?

[Roland Stigge <stigge@antcom.de>]

Hi,

http://debian-xml-sgml.alioth.debian.org/xml-policy/xml-registering-resources.html

says:
 
"Most XML DTDs should be registered in both XML and SGML catalog
systems."

Preparing the MusicXML Debian package (see
http://bugs.debian.org/259236), I wonder if (and why) this is really
needed.

Thanks in advance for any hint.

bye,
  Roland