Quote Originally Posted by Splash!
Nothing that we have seen suggests that Haki can nullify logia abilities. Even at Rayleigh's level (the level of a man who is been to the end of the world and back), the only thing he has gained is the ability to land a physical hit. Blackbeard is the only one who can NULLIFY.

Why does everyone forget that there is more to a logia user than the ability to let attacks pass through? So what if they aren't physically invulnerable anymore, they can still control an element. This gives them a reason to get better at using their powers. Being untouchable doesn't really give you much incentive to get stronger.
Of course we`re assuming haki let`s you hit a logia user and as i said, it becomes a race to reach that logia user.