About the only advantage would be if you cannot get Si readings at higher than trolling speeds on your boat and you bought and could install the inside-the-hull version of the QuadraBeam (XP-9-QB-90-T). You would be able to get sonar readings from a wider (left-right) area than you can with the standard 200/83kHz 20/60 degree DualBeam portion of the HDSi transducer.