1. I was trained to always announce "on your wheel" when taking a draft from someone from whom you haven't previously arranged it (such as a paceline, echelon, or similar). Common courtesy and important for safety.
2. Even at one bike length back, there is still a noticeable reduction in effort - less than when fully wheelsucking, but still there. And if the lead rider taps their brakes unaware of a rider behind, that bike length can get much smaller in a hurry.