I've used yeast nutrient in starters for years. All that time, I didn't add it to batches, based on the thinking that an AG wort should have all the nutrients it needs. About a year ago, I started thinking that I was kinda disappointed in a lot of the beers I was making and went looking for things that might improve them. One of the things I tried was adding yeast nutrient to the beers I was making. I have no empirical evidence, but I swear my beers have been better (flavor and attenuation) since I started using it. At NHC last summer, I attended a talk by Dr. Matthias Fischborn of Lallemand on the effect of nutrients in wort, especially zinc. The nutrient I use contains zinc, and his explanations described my experiences since I started using nutrients. I'll continue using nutrient because it's cheap, easy insurance.