Not so much the stitches holding their shape, but that the smoothness of the yarn is such that the interstitial space is too big or "hole-y", even with size 2 needles, so that cold wind gets through the fabric very easily. The alpaca would help fluff up the yarn a bit and close those gaps, but when I've spun alpaca I've always found that it tends to spin a smoother yarn than wool. The loft on a fluffier wool might fill in those gaps and keep the wind out better. Maybe not, though.

Sorry, I'm probably not being too coherent tonight. There are probably much, much better ways to describe what I mean.