Multi-mode would be heavier and bulkier. For the drone every gram saved on the spool is another gram you can add to the explosive. The article does talk about these drones having relatively small payloads due to the weight of the spool.
I briefly thought about having the drones spool out a pair of thin copper wires instead and having very high voltage running through the wires so the drone didn't have to carry a battery, but realized this would be wildly impractical as the resistance of the wire would kill the voltage and they would constantly be touching each other in the wind or sagging down to the ground and shorting out.
