Is fiberglassing a wooden hull a good, bad or indifferent thing? I'd think it would keep the water out, but then I would also promote a lot of rot, and make rot harder to repair. Drying out the boat, it seems, would help fight rot, but wouldn't that shrink and loosen the boards, and generally weaken the boat?
If it's conventionally carvel-planked it's generally a bad thing.. and done to add a few more years to the life of the boat. IMHO, it's only really worthwhile for vessels of historic significance (where repair cost isn't as important as keeping the boat afloat).
Of course to work at all the hull needs to be completely dried out otherwise the 'glass won't adhere properly and could simply fall right off (yes, I've seen that happen!)... but the biggest problems come later on if any water at all is allowed inside through deck leaks, over-active stern glands, hitting anything or worse - rain. This is because it's the natural swelling of the timber that keeps a wooden boat 'leak-tight', but if one side is held immovable by a fibreglass sheath, the force of the expanding (damp) timber and general movement in heavy weather can shear the planking along inside of the sheathing allowing moisture to penetrate between the sheath and the hull, rotting the boat from the outside in. Eventually all that is left is a roughly boat-shaped fibreglass 'bag' containing a pile of rotten timber - a sorry sight indeed!
With a sheathed hull, it is also very difficult to detect rot occurring since the planking appears quite sound on the inside right to the end and once 'sponginess' in the sheath is detected on the outside (eg. during an annual haul-out) the damage is usually too extensive to be worth fixing.
Sheathing usually isn't a problem with plywood yachts though, because (a) plywood is made of layered veneers, equally strong in different directions, which doesn't shrink/expand in the way timber planks do and (b) for simplicity of build there is usually only one or two hull joins (seams) which are held rigid by design.