These voices are most valuable. As I thought that such dissonances are obligatory for embellishment and voice with singing formant includes dissonant partial in a.m. area (see the topic "Are there dissonances in male beautiful singing voices?" for explaining) I tried to find the possibility of such dissonances in structure of soprano voice. By singing formant these dissonances arise thank to high overtones (roughly greater as 10) in area 3 kHz, where the ear is most sensitive. By soprano voices this diapason corresponds roughly 5 harmonic, hence relative distance between successive overtones are roughly 3000/5=600Hz what is essentially greater as critical band of cochlear in area 3000Hz, so successive overtones aren't able here to call essential beating, that is, sensation of dissonance. Beating is periodic alteration of loudness and it is possible to suppose that vibrato creates analogous effect. Indeed, in considered frequency diapason relative broadband of cochlea filters is small and small alteration of frequency of vibrato voice are able to call essential alterations of perceiving loudness. Such frequency alterations (deviations) are natural by support of note's fundamental frequency (pitch) and may be valid for good vibrato as result of natural talent or training.
I hope that is may be interesting.
Yuri Vilenkin