Taylor & Brown (1988) argued that several kinds of irrationality are good for you — for example that overconfidence, including the planning fallacy, protects you from depression and gives you greater motivation because your expectancy of success is higher.
One can imagine other examples. Perhaps the sunk cost fallacy is useful because without it you're prone to switch projects as soon as a higher-value project comes along, leaving an ever-growing heap of abandoned projects behind you.
This may be one reason that many people's lives aren't much improved by rationality training. Perhaps the benefits of having more accurate models of the world and making better decisions are swamped by the negative effects of losing out on the benefits of overconfidence and the sunk costs fallacy and other "positive illusions." Yes, I read "Less Wrong Probably Doesn't Cause Akrasia," but there were too many methodological weaknesses to give that study much weight, I think.
Others have argued against Taylor & Brown's conclusion, and at least one recent study suggests that biases are not inherently positive or negative for mental health and motivation because the effect depends on the context in which they occur. There seems to be no expert consensus on the matter.
(Inspired by a conversation with Louie.)