The eye of the mantis shrimp is likely the most complex visual organ in all of the animal kingdom. Each of its parts must be present and function with the other parts in order for its vision to render polarized light and multispectral images. Therefore, an ancestor of the mantis shrimp with only a few of those parts would have gained no survival advantage.
Come on!
Do you see how terrible this argument is? It's presuming that say, a simple eye that could, say, only perceive vague shapes in low resolution in black and white, is useless. How obviously, flagrantly, false.
But, we have to find a necessary assumption for this terrible argument.
To keep this argument alive, we must establish that for parts of the mantis shrimp eye to have aided survival, it must have helped it to render polarized light and multispectral images.
Imagine if we negated this assumption. So, parts of the mantis shrimp eye that could not render polarized light and multispectral images (in other words, the "simple eye" that could only make out vague shapes) could still aid survival. Well, yeah, now the terribleness of the argument comes to light.