YY to the headspace thing - couldn't have said it better myself!
There is the other perspective though, that if both parents work (even if one is part- time) there is always going to be the inevitable "juggling game" (eg. which parent can take time off for this or that school event, holidays, etc) which can equally lead to conflict and resentment - particularly as, most of the time, it will be the woman who tends to be the one to compromise here. It's still stress which affects headspace - just a different kind of stress. Some people juggle better than others. Some people are more resilient to the day-to-day "humdrum" of being SAHM. To be honest, many (most) jobs are pretty humdrum too, unless you're one of the lucky ones who has found their vocation or something.
Of course, this does avoid the scenario that one person's career continues unhindered while the other's is non-existent.
As long as the woman does not feel undervalued or financially controlled by the DH, his career progression will obviously benefit her as well. Obviously, she will be entitled to half of everything in the event of a separation and, in many cases, would still be better off financially than if she was working.
As someone said above, for many people, the SAHM model just makes financial sense.
Also, not everybody has the kind of DH who has the work flexibility to be able to take up some slack at home, even if he had the inclination to do so.