cheap dog food

Question
we've been feeding our 2 girls a dry kibble mixture of Pedigree, Iams, Eukanuba, and/or Alpo as their main diet. We usually mixed at least 3 of these together at different proportions. We add give additional ingredients (it's usually whatever we prepare for dinner - uncooked meats and vegetables). The girls are in great shape. Do you see any reasons why we should give them "premium" or organic or holistic foods?

Our two girls are rescued dogs, about 28lbs each, 2 and 3 years old. They've been with us since they were 6 months old. thanks

Answer
I have never been able to find any proof that the premium, holistic, or organic foods are any better for dogs.  I have asked those feeding them for proof, and most of them can't give a logical reason to back it.  Dogs digest very little of raw vegetables.  If they do get anything out of them, it could upset the balance of their diet.