No offense intended, but you don't really know what you're talking about. First off, there's absolutely no point in focusing on anything but the project at hand--financially or creatively. Focusing on anything else but BL would only hurt the creative process, and why would they want to start something new? Also, knowing "techniques and tricks" won't help with anything. Plus, they still have the write the damn thing, which takes an ungodly amount of time. Most of all, though, you're suggesting that they either make a point and click game and/or some done-to-death-formula game: two things DF will never do.
No offense taken, but I believe in an article Tim Schafer said that he wrote the majority of the writing for Psychonauts in just three days. I never said that they have to split their team that is working on BL. I agree with you, Double Fine should focus on BL because that is their major project. But everyone isn't working on BL the entire time the game is being made. Once the writing is done what do the writers do there? Why would knowing how to make a game (techniques and tricks), not help with anything? And how do you know that Double Fine will never make a point and click game?