I use it all the time (The Apple adaptor) and experience ZERO battery drain. I use a Macbook, though. Maybe Macbook Pro's have a slight battery drain.
I had the same experience with my MacBook. But since I got a MacBook Pro, I have found that using the power port usually only slightly extends my battery life (even with brightness turned down).
Has anyone found a way of reducing battery drain when the the MacBook Pro is connected to the power port?