I know they're not everyone's favorite, but I really like the Vahki. And I know that in lore they're supposed to be police drones, but looking at them as much as I do, I can't help but feel like they were originally designed to be labor drones.
Their staffs look like tools for handling heavy and dangerous material, cutting debri and so on. The flexible nature of their designs, and the fact that they can speak and understand Matoran speak, their speech is just sped up to be distorted.
It seems like that in their development they were changed to be enforcers, with disc launchers added.
Whether this change was made by Dume or Teridax, or seemed necessary with the City's lack of Toa, I think it's infinitely more interesting if tools to help Matoran were turned into tools of oppression rather than simply built for that sole purpose.