I vaguely recall an article that you may find in the archives that provided the information you may be seeking.

Apparently, if my aging memory serves me, myelination occurs with ageing and prevents further neurological development and hence inhibits this old dog from learning new tricks and, perhaps, recalling details correctly.

Then again, I may have it back to front so you'll have to check on this.

Thinking in terms of positives and negatives is probably not the best way to go with mylenation.  Mylenated nerves carry out different tasks than unmylenated nerves do.  All your peripherial nerves have mylenation to keep them insulated, boost signal strength and support them in repair.  Unmylenated nerves are found in the brain - here, the neurons have different supporting cells such as glial cells.  Unmylenated cells can make connections with many other neurons and I suppose you could call this an advantage with the corresponding disadvantage that mylenated cells don't connect to as many neurons.  That means that mylenated cells are less flexible, too, but you wouldn't want unmylenated neurons in your periphery - various disorders damage the mylin sheath, such as ALS, causing all sorts of movement related problems.