attributes containing subordinate elements

The TREX spec allows patterns for attributes 
containing subordinate elements.   For example:

<attribute name="foo"><element name="bar"><anyString/></element></attribute>

I see some values in allowing attributes to contain subordinate elements.
However, this is not allowed in XML 1.0.

One could argue that such patterns are useless but not harmful at present, 
and it may become useful in the future.  One could also argue that such 
patterns are not required, questionable, and should be postponed.  I'm in 
the second camp.

Cheers,

Makoto