I’m sp-last, and I completely agree with the idea that I need a partner who has strong and valued Sp, particularly when it comes to raising kids and maintaining a household. I’ve had partners who were Sp-last, and it was fun and games and self-indulgence all the time, despite my preference for having a stable home and family.
I believe that in order to have a particular outcome, a person has to be obsessive about it or they will lose focus. Being Sp-first or second ensures that one person is obsessively focused.
*EDIT*
I’m Sx/So/Sp, and I’m obsessively focused on maintaining a good one-on-one relationship and on maintaining an extended So relationship with a lot of people. I don’t care so much about Sp material security, since my security comes from the first two instincts.
But this means that my finances, home maintenance, meal planning and future job security can appear to be chaotic sometimes. (I deal with this myself by having good reserves, so inattention to these things doesn’t result in my little airplane hitting the treetops on its up-and-down excursions.)
I find that I’m subconsciously attracted to women who demonstrate strong Sp, probably because I know they can cover my weaknesses.