The issue is that there are enough different strains of yeast that some can really affect the taste of the final product. I know mostly (but still not much) about ale and lager yeasts which act quite differently in the “wort”, and the books I had on beer making said that using regular bread yeast would give too bready a flavor.
So while there certainly is sufficient yeast on the skins, it’s still a good idea to get in there ‘fustest with the mostest” by pitching with powdered yeast or wine from an earlier vintage to dominate the yeast population. One is going for predictability.
Thanks! I’ll keep that in mind should I decide to become a commercial vintner! ;-)