this is suggested frequently in these threads. it is already used in the very limited application where it is useful.

https://en.wikipedia.org/wiki/Magnetorquer

"It is also impossible to control attitude in all three axes even if the full three coils are used, because the torque can be generated only perpendicular to the Earth's magnetic field vector."

the non-earth orbiting mission i worked on didn't have any; it used little thrusters to desaturate its reaction wheels.