Without bottles and pacifiers, nursing babies are simpler to carry along. Breastfed babies are healthier. Formula gives no protection against illness, so formula-fed babies are more prone to diarrhea, ear and respiratory infections, skin problems, and pneumonia. Formula-fed babies tend to get sick more often and recover more slowly than normal. And sick babies are not much fun.
Formula affects longterm health, too.
How a person was fed as a baby affects his or her risk of childhood diabetes, childhood cancer, ulcerative colitis, obesity, and perhaps heart disease. Without the longterm jaw action of nursing, there is more need for orthodontia and speech therapy. There may be an IQ difference, too. Formulas are simply not complete foods. They lack hundreds of enzymes, hormones, anti-infectives, and living cells that are found in breastmilk.