barred2rock
Crowing
Um... because I don't want to spend the $8. I don't want to deal with Kefir... though I have to be honest here and state that I've never tried it. I'm not lactose intolerant... or if so, only mildly so. I don't buy milk to feed to the chickens. When we have more milk than we consume, and I've not frozen the excess, the sour milk goes to the birds. I'm not spending a single penny out of pocket, while giving them a bit of extra nutrition. My birds get FF, so they get the benefit of probiotics there, and if not there, then from their DL.
Years of benefit for $8 doesn't seem like much. But whatever floats your boat.
P.S. You could use the old milk with the kefir grains to produce more Probiotics.