why to not add attribute defaults to schema

http://www.w3.org/TR/2001/REC-xml-c14n-20010315

See section 1.1 where it summarizes what the spec requires:
> Default attributes are added to each element

This is covered in more detail in section 2.1:
> The input octet stream MUST contain a well-formed XML document, but 
> the input need not be validated. However, the attribute value 
> normalization and entity reference resolution MUST be performed in 
> accordance with the behaviors of a validating XML processor. As well, 
> nodes for default attributes (declared in the ATTLIST with an AttValue 
> but not specified) are created in each element. Thus, the declarations 
> in the document type declaration are used to help create the canonical 
> form, even though the document type declaration is not retained in the 
> canonical form.

So, I think we probably don't want to add attribute defaults to the 
schema.

-Greg