Primaloft told me to sew 3 inch squares - rows of stitches in both directions
To me, the rows of stitches that parallel the fibers of the insulation won't do much, because you can easily pull apart the insulation in that direction so it won't do much
Maybe they're just being conservative. If they said to sew across the fibers of the insulation, some people would mis-interpret and sew parallel and it wouldn't do what it's supposed to and people would complain
Maybe they say 3 inch spacing because some people really abuse their garment, and if you're more careful you can get away with wider spacing.
Maybe with narrower spacing, after a few years, the insulation would be a little more uniform, but not a big deal for normal use