The short answer: I'd miss them too much. That and the whole feeding issue.
The long answer:
Biologically, mothers and babies belong together, as one unit. The mother remains the primary habitat for the baby beyond the months of pregnancy. Her body continues to completely nourish the baby for at least the first six months, and then should continue to be the main source of nutrition for at least the next ix months. The baby forms its primary attachment to the mother by being in close constant contact with her.
One of La Leche League's ten belief statements is as follows: "In the early years, the baby has an intense need to be with his mother which is as basic as his need for food." So it is not just about nutrition. Nutrition may be what helps begin the bonding of mother and baby: baby must eat frequently, and mother keeps him close. they experience skin-to-skin contact through nursing, and they begin to form a bond. The baby is learning about love and life from his mother - who he thinks he is actually a part of - through the physical closeness. He is learning to trust, and that consistent person being there all the time for him brings him security. This is what "attachment" is all about.
A baby first becomes attached to his mother, and she to him, in these early months. But the LLL statement above said the early years. This means that baby is not suddenly ready to spend a lot less time with his mother once he hits the end of his first year. The statement is vague on the exact number of years since that will vary from child to child. Independence from the mother is not something that can be forced on a child - it is a gradual process.
Personally, it seems to me that a child any less than about 18-24 months old is not going to understand about long separations from his mother. Some mothers have to work eight hours a day - yes, this is a reality for some. But it is not ideal, in my opinion. As unpopular as it may be for me to say this, I think babies and young children should be at home with their mothers. This is the person they were designed to be with for the vast majority of the time in their early years, and it has been shown to be optimal for their well-being.
Some people say that a mother who will not release her baby or toddler into the care of others is being "overprotective." They try to make it about the mother's needs more than the child's needs. So many mothers are brushed off, saying, "Oh, he doesn't need you - it sounds like you're the one who is needy!" True, a mother does feel a need to be with her nursing baby, because of the need to feed him, and also because of the bond that is there. But most mothers - myself included - are genuinely concerned for their baby's well-being in wanting to keep him close. This is not to say that other people are incompetent and can't care for a baby properly (like great Aunt Ethel who's been after you to keep the baby for a couple days while you go out of town)... just that the mother believes that it is in the baby's best interest to remain with his mother.
This reaction is somewhat understandable in our culture. We are basically taught through our experiences that toddlers, even infants, who cry for their mothers are spoiled and trying to manipulate them. That mothers are replaceable by any child care provider, at any age, and for any length of time. That toddlers who don't fall asleep alone are going to have horrible sleep problems in the future. So, I don't fault people who say, "You need to get a break from that baby!" They usually don't mean any harm - it is just ingrained in our culture, the same way any old lady who sees you with your baby in a store will ask you, "Is he a good baby?"
Babies lack object permanence. When a baby's mother leaves, he does not understand that she will be back. His brain isn't developed to the point of knowing that once she is out of sight, she still exists. It as if she has ceased to exist. The longer the separation, the harder it is on the baby. The baby can even begin to mourn for his mother as if she were dead (I wish I remember where I had read that so I could quote it directly). Even as babies gain object permanence - realizing that things still exist even when not in sight - they still think they are a part of the mother. They don't begin to really view them....MORE