As good as the AI has gotten in advanced in games like Half-Life it still pales in comparison to playing a real human being. Anyone that has played Quake against the computer and has then played against an opponent on the network knows that there is just no comparison. I can't tell you how many times I was able to trick computer generated monsters into walking into a continuous field of fire. This is not nearly as likely against an online opponent. It's naturally far LESS likely in real life.
People tend to be a little more bright.
Second, lets talk realism for a second. If you are to hone your shooting skills in a video game the videogame by its very nature MUST be as close to the real thing as possible or it wont be much of a training aid. Those skills listed above are mostly weapon operations skills. Lets talk true shooting skills. Real shooting requires it for true accuracy.