If you use only 2 of the pins on a variable L-Pad, it becomes a simple variable resistor so as you increase or decrease the "volume" you are affecting the XO point and slope. I have no idea why anyone would do it this way but hey, I'm no expert.

If you hook up all 3 you are maintaining a constant resistance (impedance) in the circuit and not affecting anything but the SPL of the driver. If you go to the point of buying a ready-made L-Pad, one would think you would make use of it, no?

This has bothered me in the past so I may be sounding a bit harsh but I really don't understand. Perhaps someone can enlighten me.