Playtesting is needed. There is a reason it is called quality assurance in the video game industry, and why games are doomed to fail if the developers pay no attention to the QAs and their reports. Playtesting is used for a vital number of reasons - to iron out bugs and flaws in gameplay, to observe how people react in an environment and other such variables that designers would not be able to get at first glance. I'm reminded of in the commentary for Half Life 2: Episode 2, one of the mappers talks about how a part of the map early on in the game had an intersection where one of the playtesters would always go around in a circle, prompting them to remove that path and forcing the player down a more linear path to the destination.
That is just one example.
That is just one example.