Ah I understand it better. So you're wondering why
people would have
mention "marking" without implementing XEP-0394? I actually agree with you,
I think that this was suggested (not by me initially) as you say because
people don't want to implement the whole marking thing (with bold, italic,
etc) just to get mention. However, people can do partial implementation if
they want.
A partial implementation still lends "legitimacy" to the XEP and increases
the odds it ends up at least partially entrenched in implementations,
though. And this would hardly be the first time we've done this. Should
fallbacks be part of 0394 too?
Nevertheless when I look at the actual example given, maybe 0394 could be
split in such a way that we get a XEP defining the <span/> element without
the <markup/> parent on any defined children. Then it would of course make
sense for mentions and fallbacks to use that. Instead of moving everything
that uses a span concept into the same one XEP.