I think the best argument for secondary controls is probably for when you're already using all "maxing out" the input on one of your hands. For instance, in a FPS, you might want to move diagonally, while sprinting, jumping into a crouch, and then doing a quick melee attack. In this instance, it might be easier to use
V. The opposite goes if you were already steadily aiming a sniper (with a button to zoom, adjust zoom level, hold breath, etc), it might be easier to use
V to melee someone who suddenly comes into vision, rather than a mouse button.
Someone with a Razer Naga or similar might also appreciate having lots of additional configurations, just because they CAN. PC players are big on their customizations, which I'm sure you know.
I personally like to use the "Secondary" bindings to keep from overwriting the defaults when I can.