Raisins are dried grapes. If grapes had enough ammonia, phosphorus, zinc, and vitamin Bs then we would not be adding nutrients in the first place. So short answer is that raisins are a good source of glucose and cellulose (skin). It is not a good source of nutrients. The raisins do help in improving the flavor and ABV of your homebrew, but apart from that, it has little impact on the health of your yeast colony.