The answer is it depends. Doug and I had different opinions on this so we went and did some research online. And came up with an answer that we both agree with.
The answer is for most modern laptops with modern technology keeping them plugged in all the time on its own isn't the problem. The problem is that heat is bad for batteries. So if your laptop gets warm as it's plugged in and the battery gets hot you are probably lessening the life of your laptop's battery and you want to do something to keep it cooler.
There are products out there to put fans underneath, raise it or whatever you can do to keep it cooler. That extends it's life.
In addition lithium-ion batteries, the ones that most laptops use these days, don't like to be run down to zero. If you tend to use your laptop so it runs down so the battery is dead you're probably not doing your battery a good service in that usage.
You're better off letting it get pretty low and then recharging it. There is something to be said for cycling through your batteries range of power but keeping it cool and not running it down to zero are probably one of the best things you can do for your battery - at least with the current technology.