I'm glad to see this discussed on Jezebel rather than where I usually see this: in the comment section in Kotaku whenever there is concept art with female characters. This particularly grinds my gears as its often flatly settled with the agreement that breasts are not the best design to deflect blows.
But form and function are not two choices, they are a sliding scale. If we wanted to design a helmet that would best deflect a sword, we would have to remove the neck (a huge design flaw for humans in a sword fight). A better helmet would start at the shoulder and arch up to the top of the head, like Juggernaut's magic hat.
But we don't sit around and argue why every armor design that someone dreams up has to have a helmet like the Juggernaut, because Juggernaut's helmet looks like shit and that would be boring.
I argue that breasts on a breastplates should not be summarily dismissed simply because they are they are not the best form to protect yourself. There are many historical examples of armor that we can point to where protection was compromised for form. Take, for example, this boner armor.
That sucker is just begging to get slammed by a warhammer. But if I ever rode into combat, give me a suit of armor with the biggest fucking boner you got. It could be argued that this Boner Knight would be relatively safe from sword blows as he is an officer and would be someone removed from the peasant conscript meat grinder, but you could say the same for noble women like Yara and Breinne.
I'd also like to bring up that a blow to the chest on a breastbreast plate with breasts would only deflect into the sternum if you were standing flatly against your opponent, which you are not fucking supposed to do. Arya fights side stance with sword arm forward like a fencer, and Breinne stands side stance with her left arm front to deflect attacks. In both these stances you are equally protected if your armor has tits or not.
I'm not insensitive to the fact that in fantasy artwork breasts on armor is used to sexualize characters in a problematic way, but breasts on armor doesn't necessarily always mean that. I'm not sure where the line is, and I'm sure its different for everyone, but again, let me point to Boner Knight.
I think I also stand in a place where in a fantasy setting, fun needs to stand before authenticity. After my father, a big ren-fair dork, watched Game of Thrones I asked him how he liked it, excited to enter another Game of Thrones conversation (my favorite conversations). The first thing he said was, "Why does Bronn have a kukri?" (for those of you that aren't huge nerds like me and my dad, that's the curved knife that Bronn wears on the small of his back. On planet Earth they are from Nepal). To which I replied, "I don't know, dad, why are there dragons?" And then I continued to disappoint him as a son, per usual. Anyway, point is, its fantasy, let's lay off breastplates shaped like the female chest unless it crosses the line into absurdity.