Yes, electrical activity in the brain is definitely productive. For example, neglected children show below-average myelination in areas of the brain connected to social interaction. In addition, continuous brain activity throughout life, especially in the later years (such as playing chess, reading, writing, etc) has shown to reduce or slow demyelination (which is, eventually, inevitable).

Dysmyelination, of course, is a different story altogether. Rather than being a loss of myelin sheathing, it is a malfunction, and is often connected to conditions such as schizophrenia.