First, sorry, folks,  about the double posting—I got myself a new email
program! 

Craig,

You are right that the distinction between identifying and describing is
sometimes blurred. Often we identify by describing. I guess that’s why, if
we are going to make use of the two terms—limiting (or identifying) and
descriptive—it helps to define them the way I mentioned, by whether they can
be made comparative and superlative. 

An interesting aside about prepositional phrases is that most of them do not
describe at all. Telling me that the birthday present is in the drawer, or
under the table, or on the shelf merely tells me its position, as the word
“preposition” implies. But if you tell me about the birthday present with
the silver ribbon, you have really described (and identified) it. “With” and
“without” are about the only ones that really describe things.

As for your question about Christensen, he had a whole essay in Notes Toward
A New Rhetoric called “Restrictive and Nonrestrictive Modifiers Again.” Here
are three relevant excerpts: “The problem of restrictive-nonrestrictive goes
beyond adjective clauses; the principle applies to all adjectival modifiers,
to apposi-tives, and to some adverbial modifiers, at least to final
adverbial clauses.” “The best way to frame the problem is to ask, why do we
set off nonrestrictive modifiers?” “The purpose of all nonrestrictive
punctuation [is] to head off unwanted im-plications. Conversely, when the
modifier is restrictive, the sentence makes one statement and implies its
oppo-site; and what it implies is just as important as what it states.”
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