One theory is that it reduces intermodulation distortion since any particular signal only changes half as much. Another theory is that it was to support stereo,

One theory is that it reduces intermodulation distortion since any particular signal only changes half as much. Another theory is that it was to support stereo, but as you say, the DX7 is mono. I did some Fourier analysis to see if it reduced artifacts from the time-domain multiplexing, but the effect was small. TL;DR: I don't know.