[rayiner]

> Copyleft wouldn't be necessary if everything were in the public domain.

If everything were in the public domain, nothing would stop you from forking an open-source project and distributing only binaries of your modifications. The precise difference between the GPL and say the MIT/BSD licenses is that the GPL precludes that. Copyright is the only hook you can use to compel creators of derivative works to distribute user-readable source.

[devishard]

That's true, but for sufficiently large organizations, it would only be a matter of time before the code associated with the binaries was leaked, and without copyright, you would simply be able to use that leaked code.

Under MIT/BSD you can't, for example, use and distribute the changes to the Mac OS source code which were made after they forked from BSD.

There's still a bit of difference between GPL and just having everything public domain, I agree, but the difference isn't nearly as important as you're thinking it is.