Why? I don’t understand the logic of what you’re theorising. If your right side is damaged, why does having your ipsilateral side help? How common would this have been from an evolutionary perspective? And if your right side is damaged, and it’s taken out your right brain, now you’ve got a right side that doesn’t work and a left side you can’t control

I kind of get the idea: if damage is coming to one side of the body, then the sensory data reporting it would be better handled by putting the brain half dealing with it on the opposite side, to give it a chance to react rather then also be damaged.

But, I think the visual processing explanation is more compelling - since it seems unlikely this scenario would arrive in enough non-catastrophic situations to be evolutionary significant.

But visual processing theory doesn't make any sense with those worms, for instance

That was my intuitive first thought, but a healthy left brain stuck trying to control a crippled right body is a worse situation than a healthy left brain controlling a healthy left body and merely deadweight on the right.