Why can't a quality charger be used?
Having one, I think you will quickly see 2 [or more, I have 4] batteries are needed because you'll be burning through them relatively quickly. That's just the nature of this vape. Having multiple batteries is not meant to support, as you cited, charging a battery while the other is in use. As in the instructions, the only official charger is the vape itself (connected by supplied cable to a USB port/outlet). You can use the vape while its charging, but you can't use a 3rd-party battery charger. You'll likely be charging batteries in the unit during its downtime. Their custom-made batteries are only $7 each, cheap.
The Grasshoppers work extraordinarily well as nearly pure convection vaporizers, with convection vaporizers (stuff heated by heated air) better in my view than conduction (stuff baked in a vented chamber). There are few worthy portable vaporizers as pure convection and none as powerful as this.
Hopefully, soon enough budelee will give us first responses after trying it out.