The idea that all actors in an economic system are rational isn't a good one. A good example: the current stock market. Those people are really good at finances, they have an idea of what's a good stock and what's a bad stock. Emotions are playing a big part right now though, which causes the market to fluctuate in what I would call and irrational way.
The assumption is crucial though. Anyway it's made in aggregate & in a theoretical perfect market where everyone knows everything.
The thing is with this market that it's damn near perfect. It's a big enough decision to make sure consumers research it, work it out on a piece of paper, hire consultants to help with the decision. There's plenty of legislation ensuring information availability & simplicity.
