I've read Don's articles in magazines etc for over 20 years, he always has excellent information and seems to know what he's talking about. But, I've been in touch with him a number of times about Magic Sinewaves over the years and never seem to get a straight answer out of him, regarding if anyone is using them, any actual implementations, efficiency numbers, etc. My own research has found no real implmentations in existence either.
Best I can tell, they should work well for a fixed freq output or perhaps through a range of fixed output freq., but I'm not sure they could work well for a complex output like the comparisons to a Class D amplifier state.
Thus I think things likebrushless motor controls could possibly benefit from them, in that you could reduce the number of switching "events" necessary when compared to something like a normal PWM output. This comes at the expense of requiring very accurate switching timing.
If they add even 5% efficiency to motor drive systems, I could see them being worthwhile for things like increasing efficiency of electric car drive systems or other similar AE systems using battery power. Just tough to determine on paper if the benefits would outweigh the additional expense of implementation.