My feeling, and my observations since I've had a central nervous system disease, is that people who are basically healthy and those who are not have entirely different b12 needs.
While B12 may not be able to cure nerve diseases and conditions, it does have benefits... but for them to be felt in very bad cases it has to be a lot more B12 than that needed by a relatively healthy individual.
Most people are reluctant to try the higher amounts, though.
I feel that if people tried, kept notes, and then decided for themselves if they had benefitted that it would be a lot better than just taking anyone's word for what they should do.