[john70t]

If the baffles don't extend to the floor and seal, spray or cellulose insulation will get past and block them up hidden inside.
(That is a crucial step and often over-looked.)

Rot:
If the floor of the attic is insulated don't bother with the underside of the sheathing.
In fact that might be very counter-productive!

Unless the attic space is completely sealed...and there are both supply and return furnace vents to it..........that space is considered "unconditioned" and open to atmosphere.
The conditioned envelope is the living space.

Summer sun on the roof heats up the attic space underneath. It probably has a ridge vent or gable ends to exhaust the trapped hot air with soffit intakes under the eaves to bring in fresh cooler air. This flow removes attic moisture which would collect and pool to damage/mold/freeze/etc. Anywhere there are hot-cold surfaces next to each other, vapor wants to collect from "dew point". That's why there are carburetor heaters to keep them from freezing up.

If that space was completely sealed tight it would literally expand like a balloon. Even steel tanker trucks have crumpled up like aluminum cans when drivers forget to open the vent.

Adding insulation will make the attic even hotter and all the wood flex more. Some of that would radiate inside and increase A/C costs.

(edit: Nevermind. Just saw the next post. That is totally do-able. You do need vents to move air, and low collar-ties/exposed beams to hold the walls together..)