>My contention would be: It's pretty rare for both person A and person B to be able to earn so much under the second configuration so as to break even with the first configuration (as measured by hosehold disposable income, i.e. after taxes and everything).

This depends on your timescale. If you look at the tax returns of years 0-5, your contention will look spot on. If you compare the tax returns of two couples -- with double income in all years vs single income for years 0-5, you may find that in the long run, given the sacrifice involved in taking a career break for 5 years, it is in the favor of the couple to shell out in big in those 5 years.