So you're paying $9/week for a device that can charge your phone? Am I understanding that right? If so, that seems colossally stupid.
It's not quite like that. It's a) not a weekly fee, and b) it doesn't stop working a week after you've bought it.
It's basically the same as an external battery pack, except instead of charging that pack from the wall, you buy a cartridge that contains enough energy in it to charge a "nominal" device" for normal use continuously for a week. If you use it as a back-up for when you are, say, out camping, at a festival, going mountain-biking through the National Parks, etc, not near a handy mains socket, the cartridge will last a lot longer. That energy isn't going anywhere! If you're using your phone as a sat-nav through the day, having a back-up that is not reliant on having a mains socket isn't stupid, it's common sense.
The £5.95 per cartridge isn't bad. It's the initial price that needs to come down to get wider adoption, but can only happen over time as this sort of thing becomes more widespread. The idea of being able to hand in your old cartridges for new will also help wo keep the costs down, as they'll be able to re-charge them and put them back into the supply chain again and again.