The ventilated rib was firstly designed to dissipate the heat waves that screw with a target shooter's sight picture during high volume shooting. With enough rounds down the tube those heat waves start distorting your sight picture so you have a tendency to lift your head off the stock of the gun just enough to see above the distortion...that in turn will cause you to shoot above your targets and miss. Because the vent rib is only attached at certain points it remains cool and deflects those heat waves so you can keep your face firmly into the stock and stay focused on a clear target.
Today, vent-ribs are installed on just about every shotgun simply because of misconception and for looks. Shotguns are pointed, not aimed. If your shotgun fits you properly it will naturally point and shoot where your are looking regardless of whether or not there's a vent rib, one bead or twin beads sitting on that barrel. For the average shooter/hunter, the only thing a vent-rib and twin beads are good for is to quickly determine gun fit. Grab a shotgun, close your eyes, mount the gun to your shoulder as if you are shooting at something and then open your eyes. If that shotgun fits you everything will be lined up, the beads will form a figure "8" or the mid bead will hide the front bead. If it doesn't fit you, nothing will line up. When this happens, you MUST adjust length of pull, cast and/or drop of the stock until it does fit you. Once you get the gun to fit you could actually remove the beads and the rib and believe it or not you WILL actually shoot that gun just as well as if it where all there!!!
When it comes to serious shotgunning on moving targets, if you are focusing on the rib, mid bead or front bead you will not be focusing on your target...and you WILL miss more often than not! Keep your head on the stock and your eyes on the target!!!
