McCain is the leading GOP voice on Afghanistan.
One of the people who has taken shots at Stephanopolous, Steve Benen, gets at how bogus that is of an argument.
But there's no reason to assume that McCain is the "leading GOP voice on Afghanistan." Not only are there plenty of other Republicans who approach the issue with the same perspective, but McCain has never demonstrated any particular expertise on Afghanistan -- on the contrary, he has a record of confusion on the war. During the presidential campaign, for example, McCain was both for and against sending additional U.S. troops to Afghanistan. His most noteworthy contribution to the debate was arguing in 2003 that "we may muddle through in Afghanistan," whatever that means.
If McCain is a "leading" voice on the conflict, it's only because the media keeps calling on him to talk about it. It's entirely self-fulfilling -- the media gives McCain the stage, and justifies the decision by pointing to how often he's on the stage.
But you see, Stephanopolous will never have to address his faulty logic behind inviting McCain on again. People will just assume that because McCain was a prisoner of war that he actually speaks for the Republicans on any military issue. The truth is there are some Republicans (probably for shady reasons) who are against sending more troops which McCain is decidedly for. And McCain was a leading voice on getting us into the war in Iraq. How he maintains any credibility on military issues says a lot about pundits like Stephanopolous.