Re:Err...yeah (Score 1)

While you are very right in pointing out that a single molecule is very unlikely to instigate so many disorders, the mechanism by which this molecule acts (assuming it is upstream of all of the issues associated with the disorders) could be extremely useful in stepping around the issues.
Biological systems are amazingly complex, but they tend to depend on very simple causal chains that have a lot of permutations which depend on the outcomes of other chains (similar to the neuronal structure of the brain). When a researcher talks about their findings being useful in treating a variety of disorders, they don't mean that you could tamper with their single molecule and fix everything, they are just pointing out that their findings represent another link in those chains.
In the case of myosin Vb, it could become the target of a variety of drugs that modify synaptic strength. Increasing the strength would be useful in the treatment of alzheimers and reducing the response to specific neurotransmitters would be helpful in blocking addiction. These treatments would require connections with other research from areas such as metabolism, neurotransmitter-receptor mechanisms and neurotransmitter behaviours.
The main reason researchers try to inflate the significance of their research like this is because funding is hard to come by in biology unless you can tie in medical relevance (no matter how tenuously).