Game Informer published the second part of their interview with John Carmack and Todd Hollenshead. Here's a short snip:
GI: What are your thoughts on episodic gaming?
Carmack: You know, it’s one of those things that sounds like a good idea initially, but I’m not sure that—it’ll probably have some degree of success—but I don’t think it’s necessarily a method that’s going to cover the majority of the gaming market right now. I think it does tend to feel a little bit—it’s all in market psychology and all of that, and I won’t claim to be any kind of an expert on that there. It would be nice if games could be developed more piecemeal like that, but it still is so heavily front-loaded to build your basic core on there. It would be great if you could go ahead and release episodes regularly after that, but I think the customers feel to some degree that if they buy a game and they can get episodic additions to it, that there’s this sense of, “Why didn’t they just put that on there when I bought the main game itself?” I think there are some barriers to overcome..