Apollo 13 had a highly predictable ending (it was history, we all knew they would get home) but it had me on the edge of my seat the whole time. Sometimes knowing the ending is actually what supplies the tension. Flight 93 would be incredibly boring for the first half of the movie if it weren't for the fact that we know what is coming. Similarly, I read a book (sorry, forget which one) a while back in which the main character was murdered in the prologue. The sense of horror it gave to the entire book was an important part of the story's power.
If I remember correctly, Jim was saying that the more effective conflicts are good vs. good, in which neither side is a bad guy per se, but where you have conflicting visions of the right thing to do driving the story. There's a lot of potential in that kind of setup.
But when it really is good vs. evil, and you know good is going to win, the question is more, what kind of victory will it be and what kind of price will be paid. Sure, Frodo saved the world, but it was lost to him. In The Constant Gardener, the MC regains his self-respect and integrity, but pays for it with his life. Of course, one could argue that this a story in which evil conquered good, but the victory was not complete. It can also be argued that the ambiguous endings John Le Carré is so fond of are something few authors can pull off and still sell books. Readers tend to like an emotionally satisfying ending, which usually means they want the good guy to win, perhaps at great cost, but still with a clear-cut victory.
I'm rambling a bit here. Sorry.