I think it's a mistake to think that the traditional mother/father roles are a genuine choice. One 'choice' is weighted much more heavily than the other.
If you're a working woman you are faced with this dilemma even before you have a child. Many employers still view you with suspicion, which means that a couple, both aged say 28, with equal qualifications and abilities, working in a similar field, will probably still find that the man has slightly more job stability and a slightly higher salary. At the lower-skilled end of the market many employers circumvent this by having short-term renewable contracts with 7 days off between the end of one and the start of another so that people don't qualify for maternity rights beyond the statutory.
This all contributes to a culture where even before children are conceived the underlying expectation is that the mother will be the one stopping/reducing work.
And we can't neglect biology. Once a woman is pregnant she'll need time off for midwife visits, ante-natal classes, etc. Men do not have the right to time off for these. Many women also need additional time off for complications during pregnancy. Men do not have this right to time off.
After the birth women are tied to their babies in a way that just doesn't apply to men because of the need to physically recover and breastfeeding. As anyone who has tried it knows, expressing BM so you can go back to full-time work is demanding, exhausting and very time-consuming, and not always possible. Some women cannot/choose not to BF and that's fine, but for those who want to, how can we justify making it harder for them or forcing them to give it up entirely purely for the pursuit of money? Because that's what it boils down to for most women I feel.
Apart from all but the most ambitious people (and I think this would apply to less than 10% of the total population, both men and women) most people do not want to go straight back to work after having a baby. That doesn't mean they want to be a SAHM. It means that they just want a reasonable amount of time to enjoy their baby, establish feeding patterns, adjust to parenthood, find a system that works for them, and then return to work without having completely sabotaged their careers. This was why maternity leave was established. There is no male equivalent apart from unpaid time off which for most families means it may as well be non-existent.
Given that 82% of people are parents, I can't believe that as a nation we haven't come up with a fairer solution to this. We have come so far by introducing maternity rights, which are great. But unless they are extended to men we are always going to find the vast majority of SAHPs being mothers because the care-giving role as enshrined in law is set even before the baby is born. And even if paternity rights were identical to maternity rights, I think you'd still find more women being primary carers because of the biology of breast-feeding etc. This is not biological determinism but simply a recognition that a woman has a lot more physically invested in her child than a man. This is no reason why a dad can't become the primary carer and I think we'd see a lot more of this with better paternity rights and a higher status awarded to the role of SAHD, but denying the effect pregnancy/childbirth/lactation has on a woman and how that has a direct effect on longer-term mother/father roles seems to me to be very dismissive about women and what they do.
With the law and biology pushing men and women in definite directions, it really isn't a true choice for many. In the austerity age even those who can see the long-term advantages to role reversals may not be able to afford to do so and have to revert to traditional roles. Their choices may not be matched by their opportunities.
I wonder if it might be better to recognise that motherhood is an important, equally valuable function in a capitalist society just as much as the parent who works out of the home.