IMO... it makes sense to stay at home when you resent going to work for what little money you get out of it. It makes sense when you would rather stay at home, then work full time or part time for 10, 20, 50, 75% of your salary. That number is differently for everyone. Depending on how much they like being a stay at home parent and how much they like their job or job potential... and whether you get interesting roles at work after they are a parent. I think we are lucky in Canada, we get to try out being a stay at home parent with our 1-yr maternity leave. You can always try out being a working parent too before you make your decision. Just remember it takes time to adjust the roles and responsibilities when you go back to work. We have just settled into a good routine at about 6 months after I'm back at work. It would have been really easy to quit after 4-6 weeks because it was hard.
Here's what we have done since we have equal earning potential, although my husband makes more now (he's older, and has chosen to contract, whereas I keep my salaried position for our benefits). We have chosen to both reduce our working hours to around 80% of full time. If I went to 50%, I wouldn't get interesting work to do at work, but at 80% I get home in time to make dinner, and have a whole day each week to spend with my daughter, and get errands done - and so does he, this makes our weekends relaxing family days :) While having more kids changes the equation, even if 100% of my salary went to child care for 4-5 years, my raises and full time earning potential once they are all in school means we would catch up fairly quickly in the following 5 years. But I recognize that a lot of people would resent working for free and not being with their children - so crunching the numbers doesn't mean anything if you want to stay home with your kids.