[somenameforme]

Minimizing moving parts, so much as possible, when dealing with hardware tens of millions of miles away, let alone with a 13 minute delay in 1-way messaging, is generally smart. And Mars' atmosphere is so thin that these rovers will never be moving any meaningful payload, so their only real use case is as a scouting type system. But they also add very little value there given the existence of orbiting satellites. Even the Mars Recon Orbiter (from 2005) captures images in the < 1m resolution range.

IMO NASA wanted to try to deal with the sort of 'oh boy... another rover' fatigue and saw the drone as a way to spice things up with some passable science arguments behind it, and a relatively minimal cost. Further supporting this is that the helicopter wasn't an initial part of the plan - it was strapped on at the 'last minute', speaking in government time. In any case, I would comfortably wager against us seeing more drones in future missions, at least to Mars.