I believe that Gentleman Dave said it best, it can be a very effective tool to use occasionally during a game to keep the opponents forwards thinking.
I would like to add two points. First, you have to have huge juevos to use the trap for an entire game. All it takes is one slow fullback to not push up with the rest of the defense and your keeper could be looking at two or three forwards advancing uncontested.
Second, as defensive as I am of officiating, this is one situation where you had better have the right AR in place. A slow or inattentive AR can completely ruin even an effective trap.