Individual sentence formations may or may not be of
enforceable
copyright, and in any case English sentences are readily shuffled
about. The GFDL isn't so viral that someone bringing a copyright case
could readily claim violation of text that wasn't in the text being
sued over. (YMMV etc.)
Yes, chances are they can dilute the viral nature by dual licensing
the parts of articles that don't have to be GFDL. It requires keeping
accurate track of what parts of what articles come from where, though.