There's tons of MagSafe battery packs for iPhones. They charge the phone continuously. There's no need to let the phone drop to zero before attaching the battery pack. There's also cases with integral batteries. I assume there's Android equivalents for various phones.

I'd say these are more convenient than extra swappable batteries. They have integral charge controllers and charge via USB. There's no need to charge them in the phone or have to buy some extra external battery charger.