Now lets talk moisture. The air outside is hot, humid and disgusting. Hell, before it gets there it will see those cold R-6 insulated ducts, fittings, etc. and drip all over. Give me a break. Venting attics in the South was dreamed up by some disgruntled Yankee pissed about the Civil War and wanting to get even.
Lets now talk about durability of shingles and shingle temperature.
Venting or non-venting a roof has about a 5 percent impact on shingle temperature and roof sheathing temperature and even less on shingle durability. The color of the shingle is more important than venting or non-venting. And temperature is less important than the shingle getting a sunburn. The biggest impact on shingle durability is ultra-violet light. UV is more critical than temperature.