I've recently become a SAHM. Last year I had a temporary FT contract. DS1 was in reception, DS2 in nursery. DH works long hours on a decent salary that we can live on, and frequently goes away at short notice. We have no local back-up. My DCs were spending nearly 50 hours a week in childcare. I had work to do before work and after bedtime. I ended up bringing DS1 to my afternoon classes when his class was on strike. We were all knackered. The costs of time and energy simply didn't outweigh the benefit of working.
Since DS2 had been a baby, I'd worked P/T, that was managable, and both DCs were in nursery. Unfortunately P/T jobs are rarely advertised and I wasn't in a permanent position to negotiate my hours down.
DS1 found the more relaxed structure of nursery much easier than breakfast club/ school/ after school club. Not working means he can do extra curricular activities and that we're not in an exhausted heap at the weekends trying to catch up.
For each family there are different costs and benefits. For me, the worthwhile job is so specific it's pretty much non-existent! It makes sense for our family to put DH's job first, but the nature of his job restricts what is practical for me to do that enhances family life. For my friend, the reliability of her income outweighs the costs of her time. We have different needs 
Since I stopped working, we are all happier and more relaxed. We have long term plans regarding post retirement income.
If I had to, I could step into supply work within weeks, but right now, not working is best for our family.