I agree with @namechangeforfriday and @Yoshinori.
I think it's incredibly risky. You can trust your spouse all you want but if they die, decide to leave the marriage, become disabled, the house burns down, they get fired and cannot find a similar level of employment etc etc...it can all turn into a horrible mess.
I also think it's incredibly unfair to place that level of expectation and stress on someone. What if the working parent wants to take a break from earning and pursue their own interests and hobbies for a while? What if they want to change career paths to a more fulfilling vocation but cannot because of the resultant drop in earnings? What if they are being bullied or miserably unhappy in their job but cannot leave until they find a similar one because the family is depending on them solely? Conversely, it is not fair for one parent to be stuck with all the parenting either.
And honestly, I totally understand the need for parents being at home in the early years but once the children go to school, it seems very indulgent to continue the stay-at-home setup. There comes a point when your children simply don't need you to be there every moment and you're not missing anything as a parent if you aren't. I also think it is important to have a focus and interests outside the home and paid work, in whatever form, contributes significantly to that. It does not have to be a high-flying corporate career; that's not for everyone but even becoming an artist or starting a small business or whatever.
I feel strongly about this because I don't think women realise how often they limit themselves because of this SAH model. I know many females who cannot leave their abusive, unhappy marriages because they do not have a dollar to do so. And their worlds have gotten smaller as their children have hurried to leave the nest and start their own lives.
So to summarise, I think it's a great thing in the early years but after about 3 or so, I think it's best to get back to work. IMO.