I always keep mine plugged in and it's fine. My wife always uses hers without the power pack and her battery life is now horrible. I say keep it plugged in as much as you can! If you want you can always remove the battery and just use mains power. You can lose all your work if you accidentally become unplugged though.
I leave mine plugged in a majority of the time, but there's a setting called "Desktop Mode" that you can turn on to protect your battery's longevity. Right click on the battery icon in your system tray and click "Desktop Mode battery charge" and turn it on. It works pretty well as I've been using my 17R2 for just under a year and have no issues with leaving the laptop plugged in.