Thing is, nuanced emotion and irrationality is often actually rational - not in the sense that the human brain thought logically through the situation and decided rationally to act irrationally, but in the game theoretic sense of producing a superior outcome.
For example, if you get angry you can lose control and do things you'd regret later (seemingly irrational). That forms a powerful argument to avoid angering someone, because you know that they might react badly even if it's not in their best interests. Thus, evolution can engineer in a defense against e.g. cheaters of various kinds, even when punishing any one cheater would be counter-productive for the one doing the punishing.
(It strikes me now that there's an argument in here about banking, bailouts, the angry public and avoiding collapse of the economy, but I'm not going to pursue it.)