Spaced sheathing does not give the roof the structural diaphragm rigidity which is needed to resist high winds (aka 'hurricanes'), plus, flying debris will go right through those wood shakes and shingles, not to mention that when the wood shakes or shingles are lost to the wind that leaves the entire attic open to the wind, which then proceeds to destroy the house.
No, wood shakes or shingles *do not* perform as well on solid plywood as they do on ventilated spaced sheathing, but, given all relative conditions, when you loose your roof covering during a high wind event, *you do not* want to lose your roof sheathing, for both wind and water concerns.
Once the solid plywood is under the shingles, installing the foam under the roof sheathing really does very little to the life of the wood shakes or shingles, but (if designed properly) it enhances the performance of the house below.
Why would you use wood shakes or shingles in Florida? Because that's what 'Northerners' want on their house, they don't care about roof life ... until the roof gets a few years old and then they go 'Wait a minute, why is this roof doing this?' - 'Er, because ... they did not use "shakes" they used "shingles" and because wood shakes and shingles do not perform as well on solid sheathed roofs.' - 'Oh, then I will need to replace my roof sooner than I thought?' - 'Yep.'
