The point of the law school exam being for anyone to be able to show a difference in people's behavior in re GPLed code versus AFL+GPLed code. How can the licenses be said to be incompatible if the supposed incompatibility causes no change in anyone's behavior?
The presence of the AFL mutual defense clause would make a real difference to people's behavior in the following case. Suppose W is under the GPL and X is under the AFL. P combines them and distributes X+W to Q saying it is licensed under the GPL. Now suppose that Q sues some unrelated party R for patent infringement about some unrelated program Z and triggers the mutual defense clause. If the license for X+W is the GPL, this patent suit has no effect on Q's right to use X+W. Q can still use it--or any part of it--under the GPL. However, if the mutual defense clause applies to X+W, then Q loses the right to distribute X+W and could be sued for copyright infringement for using (parts of) that work in a way that he has no copyright license for. That means that the conditions on use of X+W are different from the GPL. The GPL does not permit using W in such a combination. -- license-discuss archive is at http://crynwr.com/cgi-bin/ezmlm-cgi?3