Why is charging while using a feature even worth mentioning? AirPods gain 3 hours of battery life (listening time) with 15 minutes of charging in the case, vs. 5 hours of total battery life between charges. That's very little difference, and if you're doing something important where interruptions are unacceptable (like a phone/video call), you can always charge one AirPod at a time in the case (they both have microphones). In practice I just don't find it hard to believe that very many people would ever need or even use the "charge while using" feature.