That's all well and good, but the GPL is set up to address problems that the authors saw, that public domain licensing does not - mainly that they thought users should have the right to obtain the source to the stuff running on their machines.

Others like it because it's a way of saying 'you can use this stuff I've put a lot of effort into, but only if we get to see the enhancements you make'.

I know that neither of these things is a priority for some developers, who prefer things to be used as widely as possible and as unrestricted as possible, but that's not for everyone.