Allow me to help you out.
4) Leather does a shitty job at protecting the wearer against blunt trauma.
I'm not quite sure if gambesons were that expensive. Mind you, they weren't mostly layered fabrics but rather padded with whatever material was available, such as raw wool, for example. Clothes weren't all that expensive, especially because just about every woman at that time could turn wool and linen into thread, weave or at least sew clothes. Leather however takes more specialised tools and knowledge to make (no, as much as skyrim might show you it works, making leather out of fur consists of more than just scraping it a bit), not to mention that you need to slaughter at least one cow or multiple smaller cattle. It's definitely a more expensive material than clothes.
More than that, gambesons actually offer a relatively good resistance against cuts and thrusts, at least better more than you'd expect clothes to.
I thought leather was doing an okay job at protecting against slashes, though. Did you do a few field tests, Devercia?