I kind of understand not mixing food and drink. Who wouldn't use both? It would be a no-brainer. But there should be a way to do it that is not a no-brainer, and still involves some choices and give and take. My suggestion:
You can mix food and drink, but not on the same attribute.
So you could use a blue food on Max Health and Max Magicka and a green drink on Stamina Regen. Or a green food on Max Health and a blue drink on Magicka Regen and Stamina Regen. As long as no single attribute was affected by both food and drink, you could mix them.