Just in case you are unconvinced by the arguments here is my experience. I am currently in the process of removing sheathing from my recently acquired 1930's kreuzeryacht. The planks surrounding the chain plates on both sides have been rotting away unseen. From the outside the rot is hidden by the sheathing and from the inside, hidden away by fittings and stripping. The rot certainly escaped the notice of a surveyor. Below the waterline is a similar story. My own inexpert view is that sheathing is a recipe for disater. Your yacht may look fine on the outside whilst inside she is festering away unseen. I certainly wouldn't have wanted to be in a blow with those chain plates holding up my mast.