Crucial? So without a stay at home parent a child will have poor social skills and ill discipline?
No, I doubt that.
But they are highly important in a child's development. It would be interesting to see a study to compare a child who spent the first year or teo with one or both of their parents and another who got chucked in a nursery as an unwanted career blocker from a month or two old as to which would be better adjusted.
I imagine that would be a pretty much impossible study to carry out though.