We have evolved to favor heuristics over logic precisely because we have evolved to make decisions,
http://www.troyhunt.com/2011/07/science-of-password-selection.html
To make a decision is to reduce data in a problem domain to some compressed form, call it a concept or a word, then reuse that form in the future. If we had to use all data we had ever learned to adapt to every environmental change, we would keep slowing down as we learned more, eventually to be crippled by the ambiguity of the data we had collected over time. While this behavior helps us to maximize the utility of the brain system we have, it also leaves us with a glaring hole: we may fail to realize that the use of a specific word, heuristic or concept is invalid or otherwise fraught with some kind of risk.
Remember, decision-making and concept formation are one-way trips. Information is discarded. This means we had better make good decisions the first time because we might not be able to easily give ourselves second chances to reconsider.
Update 20 July 2011: See also,
http://www.michaelshermer.com/2002/09/smart-people-believe-weird-things/
