People generally give interviews to promote what they're working on. This goes for pretty much everything. Not just games.
Or for something much closer to home, look at the interviews done regularly by Kill Screen. They're not promotional. If anything, Kill Screen is trying to bring attention to games or people that don't even have representation. Now, if you want to get into a semantic argument and say that Kill Screen is just doing "marketing," I'd counter with the fact that the writers are the ones doing the digging, contacting, interviewing. It's the complete reverse of how most game journalists do their jobs. That's the heart of the difference. Whose idea is the story? Whose idea is the interview? If the answer is "the writer," then that's a lot more trustworthy than if the answer was "an agent/PR rep."
Do I know whose idea Keighley's Final Hours piece was? No. Did Valve have a big say in the content? I have no idea. But it should be clear as day. That same metric should apply to pretty much everything the gaming press produces. I understand that many times there is negotiation involved, especially for the higher profile deals. But there should at least be a strong sense that a piece was wholly the writer's or editor's idea, not something given to them or suggested to them by another company.