Many times a lot. For one, most of the time the husband does not make enough. Unless he makes 90k up, more than likely its not enough to raise a family, meet their needs, and buy a home on that alone.
Secondly, this a different time now than it was 50 years ago or more. Back then, many stay at home mothers also had farms to deal with and several kids. It truly was a full time job to stay at home, take care of the land and kids while your husband was at work.
Today, most people have 2 kids. Keeping 2 or 3 kids all day does not constitute a job. I know, I know.... But what about all the housework, laundry, feeding them, working with them on their homework, and so on. Yeah, well, working parents manage to do all that too - AND work a 40 hour a week job or more.
Most of the kids I know with stay at home mothers are just plain rotten kids. The reason why is their mother just ends up appeasing the kids so she can do all the things she wants to do during the day. I mean come on, why do you think all those soaps, and shows like Doctor Phil, and The View are on TV all day long? Its because these stay at home Moms who are quick to tell you that being a stay at home mom with 2 kids is a full time job, are actually spending much of the day sitting on their ass watching those shows. Like I say, there is nothing they do that my wife doesn't do, she just does it on top of working a full time job.
Moreover, if you don't work, on top of fostering laziness, and many times rotten kids, it also many times leads to a household that does not bring in enough money which hurts the kids future because the family is not able to provide for the kind of enrichment and future education two income families can provide.
Anyway, I think in many cases its a big mistake for both parents not to work. Especially once the kids start school. This is a different world than it used to be. Stay at home moms are not out taking care of livestock on top of raising kids while the father is at work.