The way I understand it, the GPL guards against the possibility that some non-free derivative work will prove more popular, and so crowd out the free original - "to turn free software proprietary" as the FSF states it. This crowding-out may be a legitimate concern, but it's a subtle one: it's protecting code by preventing a certain class of competition, an act which may not be in the interest of users, who would probably welcome more choices.
I can draw an analogy to "poison pill" provisions: they protect the company from takeovers, but often work against the interest of shareholders.