A perspective on real estate agents I hadn't thought of, but find very interesting, is one argued by Steven Levitt and Stephen Dubner:
Say an agent's commission is 5%, and your house is listed at $300 000. An offer comes in at $250 000 - the agent's fee is $12 500. They encourage you to take the offer, since the alternative is waiting for a buyer with a higher offer to come along.
For you, waiting for a buyer who will pay the asking price is worth $47 500 - a lot of money. But for the agent, who has to show the house during the period, the extra commission only amounts to $2500.
The argument is that, as a result, real estate agents tend to see houses sell for less then they might otherwise, because they'll sell faster and it means less work for the agent, with relatively little loss in how much they make (but sometimes big losses in how much the owner might have gotten).