Yeast must have certain important nutrients or it gets stressed and produces a lot of unwanted by-products--or it gets stuck and ferments too slowly, etc. Without sufficent Micro nutrients you cannot optimize fermentation speed. If you have sporadic fermentation problems, Micro nutrients will act as insurance. When needed, they are there and provide the yeast with the minerals/vitamins/trace minerals that are missing in that particular mash. Or they provide the extra nutrients when the yeast would have been stressed by other factors.