Ok, "Drain" might have been a harsh word, but what I wanted to say was that one could remove the battery and still run the MacBook since Charging and Power worked independent from the battery.I'm pretty sure you won't drain the battery during regular gameplay, only when taxing the entire system via specific benchmarks.
Today the battery is always used and will be charged at the same time when and if it is drained below a certain level.
This is what I meant.
If you heavily use your MB, the battery will sit in-between the charger and the consuming components, taxing it even more than with previous models.
Before the MBP Retina 2012, the whole MBP would run using the charger, not the battery.