there is higher possibility of hardware failures such as motherboard fries if u dun plug in the battery while using AC. It's because sudden variation of electricity (from the electric socket) may produce voltage peak which will damage ur hardware.
Don't you notice sometimes at home, the fluorescent light seems like blinking in short interval? probably 100ms? which u'll hardly notice.
So best is plug in battery all the time no matter what. It won't overcharge since apple battery will only recharge when the battery level drop to below 95%.
To those who think that they have zero or low cycle count after a few years of usage will make ur battery life longer. NO, the chemical inside the battery will deteriorate after a couple years, perhaps after 5 years u'll change another laptop already. So don't be stingy on using battery

read this somewhere couple years back