Lots of folks are fond of pointing out that if copyright law was removed then free licenses like the GPL allegedly wouldn't be possible. This misses the point of such licenses for those of us who oppose copyright.
Copyleft is a way of saying, "Well as long as I have this privilege I might as well use it to say 'Nobody can copy, modify, or redistribute my work if they claim monopoly privilege (copyright privileges) over it, but for everybody else, it's fair game." It's a weapon to use against the system itself.
Richard Stallman once said as much in an essay I read, but I haven't been able to find it in years and I think he may have taken it down, because I think he's clarified his opinions now and doesn't believe in the complete abolition of copyright.