The problem is they'll be trying too hard to be seen to be 'useful,' so the shirt-pulls will be spotted and lead to numerous stoppages and bookings. This could drive the practice out of the game, which would be great, but then the extra refs won't be needed, but will still be there. As for spotting disputed goals, how the heck will they see anything from the back of the goal? So it all seems a bit woollyheaded to me.