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.

Peanut butter in porridge you say, I shall have to try this! Add some honey and a banana too and you could call it "Elvis porridge".

Brown sugar + cinnamon is also quite good in Oatmeal with some fruit on top.

I think the gp's comment deserves a counter-argument, instead of just saying no.