Nutritionally, your cat requires a high-protein diet first and foremost. But this doesn't mean that whole grains contain no useful nutrition for our feline companions. In fact, whole grains can boost the fiber content of your cat's food and aid her digestion, while some are rich in fatty acids (for skin and coats) and offer essential vitamins and minerals.
The key thing to bear in mind is how much nutrition your cat is getting from certain grains; then you can make a decision regarding whether the ingredient has been added for nutrition or simply to bulk up the food. For instance, brewer's rice contains far less fiber and fewer nutrients than whole brown rice — the choice is yours!