One advantage I found that by having a battery, even though I leave my computer plugged in all the time, is that if the power goes out for some reason (blown fuse, or storm..) or there is a power surge - the functional battery protects my windows session. So if I'm in the middle of working on something - I'm protected. Everything else is off but my laptop keeps running. When power comes back on it's like nothing happened with my laptop.

Good Luck!