The 20 feet claim is a ridiculous one, so I'll cut that in half for the purposes of this discussion. Chances are he won't be within 10 feet of the wall to catch the ball that is being hit hard toward said wall so I would rather have him turn to make a play off the wall than to try to crash into the wall to make a heroic catch and possibly give up a base or two in addition to injuring himself. You are doing yourself no favors, as an outfielder, if you get too close to the wall on a ball you can't catch.