So what's the benefit then?
Protection against tooth decay? I'm not sure I understand your point. That is a pretty big health benefit, I think, not sure why you think it's not a positive effect. There are plenty of studies as to how fluoride (in the water or as toothpaste) can protect against cavities.
I will reiterate my point that fluoride levels in water is too low to be dangerous, as the WHO recommends a maximum of 1.5 mg / L for fluoridation of water, while most countries that implement it use a concentration of 0.7-1.2 mg / L. Additionally, fluoride is also naturally present in many fruits, seafood, etc. as well as many groundwater sources that are perfectly safe to consume.
edit: I think I understand what you are talking about. Yes, fluoride does not increase nor decrease IQ levels. Its main job is to protect against tooth decay. But that is still a public health benefit, no?