ID-ness in XML

Here are some questions that bother me about the use of an attribute
instead of a PI.

1)  Why should an issue that most people consider to be orthogonal to
validation interrupt validation as an instance moves between
well-formedness and validity?

2)  Given that the root element might be any element in the DTD, in
order to cover themselves, the thorough DTD creator would have to
declare it for every element. True? Does this seem counter-intuitive?
Declaring the attribute locally x number of times seems inelegant, born
out of a lack of a global mechanism.

3)  As DTDs are by far the most frequently used tool for the development
of an XML dataset, is it fair to assume that most XML documents are
valid at at least one point in their lives? Accepting that, does it
follow that the addition of the xml:id attribute has the potential to
impact on most XML documents in existence? (Every one that conforms to a
DTD.)



--
Regards,

Marcus Carr                      email:  
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Allette Systems (Australia)      www:    http://www.allette.com.au
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"Everything should be made as simple as possible, but not simpler."
       - Einstein