When B12 deficiency is doing the damage, it can be a "cure all," because when treated in time damage can be repaired. But that doesn't mean B12 will benefit everyone who is having some or all of the same problems, and those who suggest that it will are promoting old mythology that helps prevent B12 from being taken seriously. B12 deficiency has been known to be debilitating and deadly for many decades, and the condition began appearing in medical textbooks more than 100 years ago, before researchers discovered what was lacking.
Many people who are deficient will never get a diagnosis, because most doctors don't know how to properly rule out deficiency; even a doctor who cares enough to check a medical reference is likely to find decades-old information that prevents proper diagnostic procedure.
Other patients are diagnosed too late to avoid severe and irreparable damage.
This is tragic, since B12 is safe and inexpensive in very large doses. Taking B12 is not the danger--having too little threatens lifestyle and life itself.
In addition, a percentage of those with neurological or neuromuscular conditions who cannot be shown (even with the best current testing) to be deficient have been shown to derive great benefit from B12, especially the methylcobalamin type. This is backed up by solid research. Thus some exceptionally well informed physicians recommend 1000 mcg oral B12 (preferabily methylcobalamin form) to all of their patients with neurological symptoms or signs.