--and some facilities do them far too often when they're not indicated (insurance reimbursement, I'm sure, plays a part here--get that money--as many more plans will cover the more extensive surgery of a nerve biopsy than a skin biopsy at this time).

Nerve biopsies are supposed to be done along with muscle biopsy and only if certain conditions, such as inflammatory vasculitic autoimmunities or specific hereditary neuropathies with characteristic degeneration patterns, are suspected. They can sometimes confirm such suspicions, but often don't reveal causes that are toxic or metabolic.

See:

http://neuromuscular.wustl.edu/nother/bx.html