The only thing nice about having a PS/2 keyboard is that most BIOSes have an option to power on your PC with a combination of keys. I find that very handy. Also, by using the PS/2 ports, you save one to two USB ports (wired USB mouse and keyboard = 2 USB ports. wireless combo USB mouse and keyboard usually = one USB port for receiver).
Even if you're a gamer it doesn't matter, you can use either USB or PS/2 unless you have one of those uber gaming mice with massive DPI settings then USB would be good to use with that. With a keyboard, it doesn't matter either. I don't think anyone can type fast enough to overflow the keyboard buffer and saturate either the PS/2 or USB interface. You only need to use a USB port with your keyboard if the keyboard has a built in USB hub or key lighting or LCD screen (ala Logitech G15, Saitek Gamer and Eclipse).