Back on topic: I feel that haunts are specifically designed for less players than more, just as a good horror game is more effective with less players. Its easier to work in personal horror and individual moments of terror when the DM doesn't have to split his attention so many ways. With four players, you almost have the time to give individualized descriptions to each PC of how they perceive the haunt. This just isn't possible with too many players since not only are you dealing with that many extra actions (or lack of actions) against the haunt, but you're forced to give abbreviated, blanket descriptions of what's happening because of that loss of game-time. I could why some people might equate the haunt to a trap in that regard since like a trap, you're hoping the PCs that can deal with it do so, and when you've got that many actions against it, its all about mechanics and loses the flavor. In fact, if your DM didn't take any pains to describe the haunt and just went about the mechanics, it might as well be a trap. Isolation is a key component of horror, and that just isn't going to happen with that many people. Its like the reaction of two people seeing a ghost versus a crowd.