I have a number of shotguns that have mid-beads. The only time I notice them is while cleaning or oiling the barrels. A fair percentage of Syracuse Lefevers have them, so I've assumed they were a factory option, but can't confirm that.
I cant ever recall noticing any mid-bead while shooting. I suspect that if I tried concentrating on lining up or stacking front and mid-beads with a moving target, that it would negatively affect my shooting. They always seemed like a sort of trap shooting gimmick for repeated presentations to me.
