I called it semi-rigid because pictures I've seen give the impression of it having considerably less flexibility away from the joints. After all, it's not like you're wearing a regular long-sleeved shirt. Plus I'd also wager untreated leather armor would be about as effective: Poorly aimed shots with almost anything would bounce right off it essentially, where a normal shirt in-game would likely leave you with a shallow cut at best and deep gashes at worst. Granted, I base that off of adventure mode showing many attacks as not quite able to land solidly in some manner apart from body shots, presumably because of body positioning.
Course, stabbing through I would imagine as actually being, well, painfully easy. Some cloth can catch and turn a blade pretty easy, but I don't think it can quite catch a stabbing so well... Unless of course Urist meant for that spearpoint to be imbedded in his ribs, in which case it did a stellar job.
That being said, it probably needs way more frequent replacement being made of cloth, since leather armors in my experience tend to wear out quickly as well with frequent fighting, be it against people or critters, which could be seen as a balance to it - you may go through a handful of suits of metal armor every decade or two, and burn through leather and cloth armors on a seasonal basis, regardless of how effective either actually is.
I'd definitely say if it isn't in the mod, some kind of treated leather armor would be a good upgrade from gambesons and basic leather armor, since if memory serves stuff like that was pretty highly valued because it was easier to make than metal stuff but more effective than stuff like gambesons.