You really do need to unplug the laptop and run it off battery at least once every few months. I've had a desktop-replacement laptop that I ONLY ran off the power cord for three years. I literally only ran it off the battery once when I first got it to see how long it lasted, and maybe two or three times when the power went out. New, the battery lasted about three hours. After a few years of never running it from the battery, I get around ten minutes of battery life even when not doing anything. It's not a big deal for me, as I always run it while plugged in, (ten minutes is long enough to shut it down safely if the power goes out), but I was astonished at how much battery life degraded.
Sure, one example doesn't mean much, but from my own experience you should run the thing off battery power at least once in a great while to preserve battery life. Maybe the battery degrades over time anyway, but I seriously doubt it would be as bad if it ran from battery once in awhile. (Yes, maybe it was silly for me to get a laptop if it's always plugged in, but it's a lot more convienient than lugging a monitor and desktop up ten flights of stairs every few weeks.)