On-screen buttons in a navigation bar first appeared on the Galaxy Nexus with ICS.
The first mod for the Galaxy Nexus was to change that button layout. People went for that mod like crazy.
We never understood what Google was thinking - they were software buttons that you couldn't configure.
Months later, HTC ignored that trend and recreated those same 3 buttons at the bottom of the screen on the One X, expecting to conform to the Nexus way, and abandoning their traditional 4-button setup. Except on the Nexus, sometimes the 3 buttons became 4, adding the 3-dot overflow to the right, something that off-screen buttons couldn't do.
One of the first One X mods was to modify the buttons.
See where this is heading?