As nice as it is to have the guy stand up for Apple I don't think that from a practical point of view there is anything that he could say that would make a difference.

I'm not a lawyer, but as I understand US courts from reading Groklaw and HN and Wikipedia, the arguments put forward in friend-of-the-court briefs can still be helpful to judges who may want to rule a certain way, but don't know all of the possible arguments they could use.

There's only so much time they and their staff have to do research, so members of an industry and their legal representatives can contribute if the judges are on the fence or just looking for arguments.

Yes, that's what I meant, sorry for not writing clearly. The amicus curiae brief they wrote is http://images.apple.com/pr/pdf/Lavabit.pdf

Page 16 is the Thirteenth Amendment argument.