As I understand it, this is exactly what decreases the battery-life. I had to have a battery replaced last month and have been letting the battery meter disappear completely until a message comes up saying there is not enough battery power to function, please recharge.
I didn't know this about rechargeable batteries... if you recharge them at 1/2 capacity, that is the new maximum life of the battery. If you recharge them from 1/4 capacity, 3/4 of the original battery-life becomes the new maximum capacity. Does that make sense? Seems ridiculous to me, but this is what I've been told.
So, if you always have the thing plugged in, you should be ok right? I'm not sure. I would make sure that you have a full battery, or an empty battery before plugging in. Just be careful not to plug it in at 99% capacity!