Hi - I was in the same boat. I had a spinal that was non diagnostic, and then had a muscle biopsy that showed demyelination but also non diagnostic. One neurologist said I should have the nerve biopsy, the other said no - for the reason that there can be permanent damage etc. I hummed and hawed for a long time but as things continued to deteriorate (albeit slowly) and I was getting no closer to a diagnosis I decided to have one. Here is what you need to know. The procedure HURTS! They do not numb the nerve - they numb the skin so you don't feel them cutting into you but then they have the nerve all exposed - it looks like a string of spaghetti - and they cut it. The doc accidentally touched it with his scissors before cutting it and I almost jumped thru the roof so I had to brace myself when he cut it - and it was incredibly painful. But - it only lasts for a second. And of course you feel nothing when they sew you back up. The other thing you need to know is that the recuperation period is 6 weeks - so off your feet or limping for much of that time. As for permanent damage - mine is permanently numb and if you touch the area around it it is like hitting your funny bone - kind of tickly painful - but certainly tolerable. Unfortunately for me the result was - again- non diagnostic. More demyelination and inflammation but that is all they could say. So that part sucks but I don't regret getting it done as otherwise I would always wonder.
By the way - that was a year and a half ago and I still have no diagnosis so i am getting reconciled to this perhaps forever being the case....
Hope this helps