Some of the guys in my SCA barony fight in almost nothing but leather with a few strategically placed plates. Some of the leather stuff they wear is very thick and rigid. I could easily see how it would take a good solid hit with a good weapon to hurt it, let alone them underneath. Leather was very common back then, so making a good tough leather breastplate, shoulders, and arm/leg guards would probably not be too cost prohibitive.
little aside
Armor is a vary labor intensive process, I should start a “Ask the guy who is building a suit of armor” thread. I am working on my first set of armor for SCA combat.
By the time I am done it will probably be 50-60 hours of work and about $300-400 in materials. Just hammering out bowl shaped pieces for elbows and knees is a bit of a chore. This is not even full plate, there will be some leather components to it as well.
Granted I am using aluminum which is more expensive than steel but much lighter and I am doing things like hand staining my leather, mirror polishing aluminum, etc.
I am gonna look like one sexy beast.