Yeah so your cycle count goes up and Apple gets to sell you a new battery sooner
I leave my MB plugged in most of the time, 24/7. I do plan on draining the battery once a month, just a known fact it will keep the batteries charge fresh and strong.
However either way you choose you can't harm your laptop or battery. It's just a matter of what's the most life you can get out of the battery. Worst case you have to buy a new battery in a year. That's what Applecare is for.
Oh and don't run it w/o a battery at all, you will notice a decrease in performance, not to mention if your power-cord accidently pulls out your machine will shut down. Plus an idle battery is not good at all.