Lithium ion/polymer batteries do loose capacity at a faster rate if they are constantly "topped off", which is what happens when you leave the device plugged in for a long time. It charges to 100% and then stops. As soon as the battery goes to 99%, it starts again. This repetitve cycle from 99% to 100% is actually worse than, say, 80% to 100%. That said, the real-world impact is small. Rougly speaking, if a battery has a 3 year useable lifetime, it might diminish to 2.5.