It seems to me that a similar analogy can be made with happiness, where happiness is thrive mode and depression is survive mode.
People who are depressed aren't very productive. Depression is no useful survival mode.
Yvain theorizes in the post how someone who formulates a heuristic for themselves early on in their life that says "the world is basically dangerous" will become a right-winger, and someone with the opposite heuristic will become a left-winger, and this explains why people divide so easily into political categories.
I don't think that people are so easily devideable into political categories.
I can imagine plenty of situations where passively being sad and not going out and attempting to be productive are safer. Maybe depression is an alternative to cabin fever? Long hard winters are easier to survive if you're too depressed to go out and possibly freeze to death and instead stay in your cave/yurt eating the most easily accessible saved food. That would explain the evolutionary value of Seasonal Affective Disorder.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.