Generally, I'm not one who thinks preschool is all that important. Yes, the skills learned are important. Kids need to learn colors, numbers, letters, how to wait their turn, and basic classroom behavior. Any involved parent can teach these skills at home and by involving the child in church or some other program.
However, two of my children did attend a formal pre-school. Both Keegan & Dawnlynn have developmental delays. After participating in the Birth-to-Three early intervention program in our area, they each attended our local public school district's special education preschool. I can state with conviction that early intervention works.
At school they received speech and occupational therapy that I couldn't provide at home. Keegan went for approximate two hours a day, five days a week.
Dawnlynn went for the same until January of this year. She'd begun begging to stay home and crying about going to preschool. In addition, her development is at target, with the exception of a few speech sounds. Finally, she was showing a great interest in the schooling we were doing at home. Over Winter Break, Dawnlynn was much happier staying home with us. Now she goes to preschool one day a week to continue speech therapy.
As for my other children, Kathleen was a bright child who was always ahead. She would have been bored in preschool. She joined a public school strings class with no problems in 5th grade. Due to her developmental delays, Constance would have benefited from the early special-ed program. Baby Benjamin isn't old enough to make the preschool decision yet.
For a typical child, I wouldn't worry about preschool as long as Mom is playing with and teaching the kiddo. We count stairs while we are climbing them, talk about colors in the produce section at the grocery store, and read together. We also attend church weekly where the children learn what's expected when in a group.
Socialization is always a question. But, how much socialization does a three, four, or five year old really need? If s/he's an only child, there's an argument for getting some extra outside socialization. But, if there's more than one child of somewhat similar ages in the family, playing with each other and with other neighbor kids some is plenty of socialization for that age.
Finally, I don't think that preschool is bad. I don't think that parents who put their kids in pre-school are bad. There are lots of good reasons that families make the choices they do. I just don't think that a child who doesn't go to preschool is doomed.