I thought the yeast on the grape skin was sufficient for fermentation. I grant that I know almost nothing about it -- mostly something in a Sam Levenson book where he describes his parents making wine for Passover; he only mentions their adding sugar and water. (Of course, he wasn't writing instructions -- just explaining how they came out with so much as his father kept adding sugar because it wasn't sweet enough and his mother kept adding water because it was too sweet!)
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.