--as the others here have said, is really only now for very special diagnostic suspicions and indications.

Because of those indications it should also involve a biopsy of muscle tissue as well.

In general, at the very least, it will leave some areas of patchy numbness. That may be worth tolerating if you get a definitive diagnosis and a treatment plan. But, some have reported the standard post-operative complications of surgery (infection, etc.) and the fact that the doctor has a suspicion but doesn't want to tell you is rather disquieting.

Take a look at this--it's a very comprehensive look at how/why a nerve/muscle biopsy might be performed:

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

A lot of the more common diagnoses, though, might be made with other testing. In my estimation, only a few of the diagnoses listed, mostly very rare ones, quite a number of them hereditary, would require a nerve/muscle biopsy for confirmation. So it would be good if that doctor told you what was being looked for.