Passing DC through in an alternating direction IS AC. In this case it is not only AC but exhibits half wave symmetry with no DC component in the coil current (average over the "cycle" is 0. Sure, it is not a nice sine wave but that doesn't matter (and a square wave is actually better in this case). The commutator acts as an inverter in the motor as Salmon Egg correctly said.
Also, as a matter of fact, the rotor current does NOT produce an alternating magnetic field. The field is, ideally, stationary at right angles to the stator field -the maximum torque position.
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That is already happening with some fractional HP motors where speed control is required. Some of the newer washing machines with direct drive are an example. Also my electric lawnmower has a "brushless DC" motor. Large- industrial motors- probably not as the polyphase induction machine is too damned cheap, simple and reliable so why go with a more complicated device to do the same task?