What I meant by my post was that they don't need supplements or extra doses. It can actually be harmful than good.
Vitamin B3 (Niacin) can be found in just about everything they eat (what I feed my ducklings and adults).: Leaf vegetables, broccoli, carrots, sweet potato, asparagus, whole grain products, pasta, rice, cereals, mushrooms, eggs, tomato's, and fish, yep, fish!, as well as your duck pellets...just to name a few.
Feeding your ducks, especially ducklings a balanced diet in these foods will be enough niacin to keep them happy and healthy. But if your one of these people who don't feed the above, by all means, go for it, give them the brewers yeast. They will be missing out on other important vitamins and minerals.
I'm no expert, but I do believe a well balanced diet to be beneficial in any living thing, including ducks