Most S-n-G hulls aren't designed to be glassed on the inside. They're strong enough with just outside 'glass. Some early boats were built without outside glass either, just taped seams! But it is common practice to 'glass the cockpit floor, more for abrasion resistance than strength. One difficulty of pre-'glassing would seem to be the inevitable increase in panel stiffness, which might make assembly more difficult. But you're right that 'glassing on a flat bench gives better control over the finished product.

Mike
Friendship Kayak