I do that often. When I'm low and I want to listen to music, I'll put in a power bank and listen or when I'm away from a charger and I want to iFlix/Netflix again I'll put in a power bank and earphones or in the car while driving back home, I'll plug in the USB car charger and AUX cord. (The Bluetooth is horrid quality).

I won't mind as much if they had some sort of quick charge or wireless charge but they don't and that is going to be a problem.