Nobody says something like "we don't need it" - in fact I really like the idea. It's just that xml tags are part of xml and not existent anywhere else. Thus this should be defined within the syntax definition of xml. I said "plugin" because I'm not aware of a mechanism within the syntax files which does this. Also "plugin" does not implicate that it's not being shipped with sublime. There are already things that have been merged into the base packages as soon as it made sense. All this let's me think that it's best that someone throws out a working prototype and Jon could decide if and how to merge it into the xml/html packages.
I've found two other problems:
Customer support service by UserEcho