[Models] X- options on schema elements

["Vithalprasad Gaitonde" <gvithalprasad@novell.com>]

Section 4.1 of [Models] says:
"Implementors should note that future versions of this document may
  expand these definitions to include additional terms.  Terms whose
  identifier begins with "X-" are reserved for private experiments,
and
  are followed by <SP> and <qdstrings> tokens."

Why is there a restriction  that the X-<tag> be followed by qdstrings
alone. Shouldnt implementations be allowed to return binary values as
well here.

Thanks,
Prasad