Full-length battens have many virtues, including better shape-holding, more durability, and easier sail handling. The sails last longer because the full-length battens prevent flogging (the violent snapping back and forth that occurs when tacking, reefing, hoisting or lowering the mainsail). As conventional sails flog, the sailcloth breaks down, especially at the front end of the battens where the leech of the sail “hinges” back and forth. Flogging is the prime culprit in sail wear-and-tear.
Full-length battens induce a smooth airfoil shape to the mainsail, improving performance in all conditions. In light air, the full-length battens hold shape instead of the sail sagging; the battens also prevent slatting as the boat rolls in light wind and waves. In heavy air, the full-length battens keep the sail from flogging when the sheet is eased to reduce heeling: the silence is a pleasant surprise. Full-length battens also prevent the draft of the sail from moving aft in heavy air when sailing upwind.
Downwind, in light and heavy wind, full-length battens hold out the leech of the main, providing more projected sail area, which makes your boat faster.
Racing sailors need to be aware of the drawbacks to full-batten mains: they are heavier, it is harder to change the shape as the wind speed changes, and since they hold their shape so well it is difficult to see when they are luffing.