My PC hooked into my 46" Sharp Aquos via HDMI at 1920x1080 res looks ridiculous, otherwise its on a 22" widescreen monitor at 1680x1050 res. You've got a lot of people playing their consoles on 720p sets, or through component/analog cables, looking nowhere near as clean as it would in high res on a PC.

Other than that, it's more about controls being so much more precise and feeling more natural for FPS games on keyboard/mouse rather than a gamepad, especially in a game like HL2 where the native control scheme is based on keyboard/mouse.

It's all about preference, I prefer sports games and 3rd person shooters on console, and first person shooters on PC.