The GPL *does* place a burden on a third party who wants to use
       that code without the GPL, because that third party must obtain
       it from a place other than the collective (GPLed) work
       containing it.

   This is another example of a misconception about the GPL.

   If the person who writes the addition cares about the matter, he can
   attach his own copying permissions to the addition.  Then, if you
   wish, you can get a copy of the combined work, delete all but the
   addition, and use that under the terms its author set.

That is not true!  I quote from the GPL itself:

   These requirements apply to the modified work as a whole.  If
   identifiable sections of that work are not derived from the
   Program, and can be reasonably considered independent and separate
   works in themselves, then this License, and its terms, do not apply
   to those sections when you distribute them as separate works.  But
   when you distribute the same sections as part of a whole which is a
   work based on the Program, the distribution of the whole must be on
   the terms of this License, whose permissions for other licensees
   extend to the entire whole, and thus to each and every part
   regardless of who wrote it.

That section specifically requires that the GPL's distribution terms
apply to *all* of the collective work, including the addition which
someone may want to use separately.