I weigh 300 pounds, and my husband only weighs 250... does that mean I'd be more nutritious for a snake meal than him? If the Mazuri adds bone, muscle, and not just FAT weight, then I would agree, your Mazuri rats are a better meal than your friends dog-food rats. But if the ONLY difference is more fat, then perhaps the Mazuri is not the best idea.
Again, as someone mentioned, it's what works, what keeps the snakes fed and healthy. If it's not broke, don't fix it.