I thought the MBP battery switched off charging when it was full, so that it charges up the battery and then disconnects it if you are plugged in. When the battery has self-discharged some amount, it tops it up again. But maybe I misunderstood.
The flipside, though, is that Li-ion batteries also age from discharge cycles. So I really don't think it's better to run the battery down, charge it up, and repeat. The number of charge cycles is limited, so I bet that will kill it faster. If you really want to save the battery, you can take it out and put it in the fridge when you are plugged in.
At least, that is the way it is supposed to work. I've been through a few batteries that were replaced under warranty free of charge by Apple for premature failure.
I've noticed that one of my 4 MagSafe chargers is not charging correctly any more even though it can power the computer just fine. It took me a while to figure out that was the case as I kept incorrectly concluding that the problem was in the battery. There have been reported problems with the chargers failing in this way.
I think that because a modern laptop can now have intelligence in the charger, the battery itself, and the power management functions of the laptop, we are seeing problems that are hard to debug.
But it seems apple is not officially selling those batteries, which makes my gears grind. I don't want to walk to an apple store or send my laptop away just to have the batter swapped.
The flipside, though, is that Li-ion batteries also age from discharge cycles. So I really don't think it's better to run the battery down, charge it up, and repeat. The number of charge cycles is limited, so I bet that will kill it faster. If you really want to save the battery, you can take it out and put it in the fridge when you are plugged in.