It was probably over valued by Payton, but it's particularly valuable in the Shanahan outside zone scheme. The point of the scheme is to get the RBs in space against CBs and Safeties so you need WRs that can block those guys, or at least force them inside, to open up the run game. It's possible to do it with guys that are just adequate blockers but it really makes the run game go which really helps the passing game by keeping the Defense from getting to aggressive in the pass rush.