Haku and Zabuza both have shorter scenarios than the other characters. I guess they don't have any reason to fight anyone besides Naruto's team as per the story, so that's all they have.

Haku's a little faster than other characters and has pretty decent damage. His level 3 might be a tad on the weak side, but that's fine since it only has 2 parts to it. He's got a counter jutsu as well.