In this excellent article about usability, Jakob Nielsen, perhaps the web's foremost usability expert, addresses objections that some people have with usability studies.
In particular, he answers those who say that usability experts test with "stupid" users. Nielsen's response:
"Usability lessons are not always easy to take. It is a painful experience to sit in the back room watching as a user clicks every button on the screen except the one button that "obviously" leads to the answer. The first time project members observe a usability study, they almost always lapse into denial about the true lessons of the experience.
Until we bring in the second user. He or she typically has many of the same problems as the first user. Then the third user comes in, and again: many of the same problems. At this point, designers often start to soften to the users' plight."