If you look at the feet, he will kick your ass even harder.
AND if you fight a person just by looking at their feet, YOU are at a disadvantage.
IF LOOKING AT FEETS WHILE FIGHTING IS SO GOOD, WHY DOESN'T EVERY NINJA DO IT.
it just means that if they aren't force to do it, they won't do it, because its a hard task to do, and fer christ sake, Gai is fighting Itachi, just that is a MAJOR disadvantage already.

the only person that can beat itachi is Gaara, because Gaara will throw sand in his eyes and make him go blind. OR Ramen shop owner will beat itachi because he will poison the Ramen with rat poison.