Zipline's system is a much lower cost, light weight implementation of what InSitu (now a Boeing subsidiary since they were acquired) designed for the defense market, for their Scaneagle. They were by no means the first to launch or recover a fixed wing drone in that manner.

As you point out it provides a lot of advantages in not needing to carry the extra weight for a VTOL, no landing gear, extra energy boost on launch. It's very close to the optimal energy usage in watts for motor, per size of craft, that can be achieved right now.

The other advantage of using a single motor fixed wing craft is ability to use lithium ion batteries which are relatively higher Wh/kg density. These are not as capable of instantaneous high amps as lithium polymer are. The best commodity, commercially available lithium ion batteries are in the range of 210 to 248Wh/kg right now. The best lithium polymer batteries are about 178Wh/kg.

FPV hobbyist fliers who build fixed wing and flying-wing craft use lithium ion for longer range.

https://www.google.com/search?q=insitu+scaneagle&client=ubun...