If we look at Bullet-resistant protection as we have it in our modern day, typically rifle-graded plates will have the strike face covered with ceramic, which helps to dissipate a large amount of velocity to the point where the centimetre of steel behind it is not penetrated and you don't end up with a bullet in your body. But the ceramic fractures and becomes less effective as huge chunks of it are displaced. I understand your point, that perhaps the one or two HP could be considered the impact damage breaking a rib or something, but if a PC ends up losing half of their HP to impact damage, any descriptive workarounds to acknowledge the armour, acknowledge that it played a role in saving the PC some damage, but did not prevent all damage and is no worse for wear seems a bit contrived. A plate of metal being deformed from a bullet or links of chainmail broken after being hit by an axe will be considerably weaker than previously. Even a sandbag, listed as 20 armour (CR pg 112) being shot at by a thompson is going to start spilling its sand and be less effective cover.
OP, I personally don't find the armour all that relevant in classic, but just house rule during prep as long as you have a basic knowledge of the anatomy of the enemy. If we take Gla'aki crawling around his lake for example, after googling what Integument meant (tough skin), logically if a tank has 24 AR, and Gla'aki's skin has 40, no matter how many .45's he was hit with, he would not be harmed. but dropping a depth charge would bypass some of his protection through shockwave damage. Ramming him with a boat would probably deal damage depending on whether you feel he has internal organs around the point of impact. As much as I love BRP, any system is not entirely dependable for GM needs, and the core rulebook flat damage reduction for armour personally feels too mechanical and arbitrary to describe convincingly, so I decide how to handle it on the fly.