Yeah, maybe there is a small chance that a steel plate is broken, but this probably only happens when the impact vector of the blow is quite parallel to the armor's surface normal, otherwise it would just be deflected. And don't forget we're talking about dwarven armor here, which is much thicker than human armor (at least in my imagination) and of higher quality (even less gaps and cracks).
Back on topic: I've done some preliminary manual testing for marksdwarves, and it doesn't seem to be worth a full analysis, as the results were quite the same as with cc weapons (iron doesn't pierce iron ever, steel pierces iron always, steel never pierces steel, etc.). Also, giant cave spider silk clothing doesn't protect from iron bolts at all.
First, I'm not sure I like the rationale "dwarven armour is thicker", because in the game, all armour is the same, regardless of who makes it. I have no problem with dwarf-made armour *being* thicker, but that would need to be represented somehow.
Also, metals forged by medieval techniques are not uniformly strong. It takes incredibly precise temperature control, not to mention precision forging, to produce a sheet of metal with no weak spots. I think this should be represented in-game as the difference between plain quality and masterworked items - the latter should use the RAW material values all the time. Low quality stuff should use a random number between, to pull numbers out of nowhere, 75% and the full value, each time. The higher the quality, the closer that range gets to 100%.
Or, for a more involved system, Toady could write some kind of weak-point checker to strikes. Where, dependent on attacker weapon skill and defender armour quality, a weak point of the armour can be struck - hitting a joint or under a lame or something. Then you add armour quality to that on the rationale that better-forged armour has less weak-points by design and more uniform metal quality. The system could be expanded to allow small-contact-area weapons to do this more often, or to take advantage of weak points more successfully. But that would require some rather complicated modifications to the layers and parts code, I think. Let's see what can be done over the next few months by just adjusting materials and weapon values.