Most rechargeable batteries, particularly LiOn, have a lifetime defined by number of full discharge/recharge cycles. For most LiOn batteries, this is around 500 cycles. Therefore, if you cycled AC to the laptop 5 times per day, the battery would be dead in about 100 days. (BTW, partial cycles count much less against the battery lifetime.)
On the other hand, it is good to fully cycle a laptop battery around once per month. This allows the battery level monitoring circuitry to "re-synchronize" with the actual battery level.
So to maximize battery life, you should run the laptop on AC as much as possible, while occasionally running on battery for a full cycle.