Easier to Furl. Improved draft shape. Full battens induce a much smoother sail shape that holds better especially off the wind and in choppy seas. Improved profile shape. In practice this means less heeling, less weather helm and more speed.
Full battens are able to provide the support needed for a large head roach.
Must have lazyjacks to enjoy the benefits of easier furling, otherwise the full battens fall past the boom and are harder to handle than a sail without full battens. Must have telltales in the luff of the sail because the full battens eliminate luffing making it impossible to check the sail trim by watching the luff. More weight aloft. The additional weight of the battens pockets and extra hardware aloft makes the boat more tender in a breeze. It also makes the sail heavier to hoist.