A family member is a realtor, and she says people have no idea of the stress of trying to sell your own home (in certain markets), plus the fact that fisbos often do get less for their homes and are talked into unfavorable contract conditions that any decent realtor would have refused.
As for the "discount" brokerages, these are agents who provide minimal services (and often aren't very competent, because they never go into a deal very deeply). They are relying on the fact that the person on the other side of the deal will be a full-service realtor who will end up essentially handling both sides of the deal simply to get it to go through.
The old motto, "you get what you pay for," sure hasn't changed.
That said, I agree that people shouldn't pay too much, but the 6% is split between the selling and buying agent, so generally each actually gets only 3% (which is why agents like to sell a house that they themselves have listed). And statistics consistently show that houses sold through realtors sell for higher prices.