It's well documented that Iodine assists in the sheding of the carapas. Iodine helps to soften this exoskeleon allowing it to be shed. If you don't add iodine, it can be difficult for the creature to shed, leading to death.
Even my prawns get iodine added. It's not accurate to say it doesn't help and not to use it. However it is not required to be used....I started with mud crays as a kid and never used iodine but as an adult have found many uses for it. One should use it with caution and understanding of what they are doing first, before adding anything to an AP system.