This "gains nothing" is still a personal preference, as for me personally, that statement is just plain wrong.
Being able to connect the mouse, joystick, another joystick and gamepad to the keyboard is a comfort I would never want to be without.
I was without it before, and it was really annoying. I hope they produce more USB keyboards, so that PS/2 can finally be removed. The reason why USB mice and keyboards exist, I guess, that an older standard can be ditched an favour of a new one. But, as so often, ditching a standard is unfortunately hard to impossible

I never understood why PS/2 is still not abandoned, USB is so much better. Guys like you can just plug everything to the computer, like with PS/2, and guys like me or a friend of mine, can take advantage of the extra comfort that is possible.
RJ