Leaving your laptop plugged all the time is not good for 2 reasons:
1) battery is aging MUCH faster when it's warm (warmer than room temp)
2) li-ion/polymer batteries does not "like" to be in 100% charged state. It looses it's capacity quickly.
If you want to use your battery when you need it - Better charge it to only 30-40%, unplug it , and store in cool place.