Saturated fats make carbs digest more quickly. For example, if I have oatmeal I can go to bed immediately because it's low GI. But if I add a spoon of peanut butter then my heart will get a little bit contentuous and keep me awake.

As well, adding a new substance introduces the potential of allergens. I have a dairy sensitivity. If I have a grande latte from Starbucks I'll spend the next hour in a haze. I can have it, but it's worse than just sugar in many respects. But the equivalent soy latte leaves me refreshed. For some people the opposite might be true.