EdS is correct; the bead(s) on a shotgun are not to be used when shooting.
Those who like two beads on the rib tend to use the mid bead as a rear sight......bad idea.
You should not see any of the beads on a shotgun, not consciously anyway. If you have to have a "light tube", especially one of a certain color, you are aiming the gun and not shooting as effectively as you could be.
My 11-87 came with a mid bead. Sometime before the fifth time I shot it, the mid bead fell out. I can't tell you exactly when it went astray, because I never noticed it gone until I was cleaning it after those five hunts. Minor sales point is what they are, not much more.