isn't negative power also generating useful work?
Reactive power is not negative power. Positive (real) power would be defined as either power flowing from source to load (or load to source) while negative (real) power would be defined as the opposite direction. So positive and negative is just mathematical convention for the direction that real power is flowing.
Reactive power is just real power that flows circulates around the circuit but is never dissipated so never performs work. It flows from source to load but then flows back from load to source at some future point in time.
In a three phase system it could be circulating between the phases rather than between source and load. Either way it's not dissipated so doesn't perform work.
It never performs work but since it is still flowing and thus still takes up ampacity in the wires.
the current also flows through the coil of the motor, generating the magnetic field it needs to operate.
Ahhh, but a magnetic field doesn't inherently perform work just like a permanent magnet doesn't inherently perform work. Supporting a magnetic field alone is not work anymore than an asteroid hurling through space is performing work by virtue of the kinetic energy in its motion.