This trope in real life
For the most part, instances of this trope only occur in real life when the armor is purely ornamental, has some sort of ritualistic importance, or is simply not intended to completely protect the wearer.
Roman gladiator armor, for example, was less concerned with providing optimal protection than with making their fights entertaining to watch. Competitive Balance was ensured by each gladiator type having its own combination of armored parts and weak points, while the large amount of exposed skin allowed the audience to admire their muscular bodies and clearly see the blood whenever one of them was wounded.
Also, the seemingly random pieces of gladiator armor were actually well-considered. The Murmillo, for example, had a large rectangular scutum shield which could protect the left arm (that is, the shield arm) and the body from about the neck down to just below the knees. The parts that weren't covered by his shield—the head, the right arm (that is, the sword arm), and the shins—were the parts for which he was given armor. Since the Thracian had a smaller shield called a parma, he was given high greaves that also covered the knees. Even the un-helmeted and mostly unarmored Retiarius with his net and trident had a modicrum of protection with his manica for one arm and a shoulder guard with a tall upright plate on the same side. This way he could always lead with the armored side while keeping the foe at bay with his trident, slip the leg if his opponent struck low, and duck his head behind the tall shoulder plate if his opponent struck high.
Furthermore, most fights between professional gladiators were not to the death, and while they did wound each other for real they tried to make it look more deadly than it actually was. The matches were refereed, and they had access to quality medical care. It was also relatively rare for the sponsor of the games to command the winner of the match to Finish Him!. It wasn't quite as necessary to protect the vital organs of the torso because they weren't necessarily trying to kill each other in the most efficient way possible.
Roman infantry wore body armor because unlike gladiators, they would be fighting in battles without any fancy Flynning, in which everyone was supposed to stay in formation and thrust to kill. Since they used the large rectangular scutum in close quarters they were especially worried about foes reaching over the top of the shield and thrusting down into the neck, shoulders, and upper back, which is why they used body armor that covered the front and back of the torso, pauldrons, and helmets with flared neck guards. They didn't use arm or leg protection because those were targeted less often in battle, and because it might have interfered with the full kit marching and hard labor they were expected to perform. A legionary could survive and potentially recover from a wound to one of his limbs, but he couldn't afford to give his foe the opportunity to stab him in the heart, lungs, or belly for an outright kill.
Many cultures that fight largely naked are able to do so because they use shields. Armour with large gaps in coverage can make sense when the shield is the primary means of defense, but doesn't make sense when the armor is expected to be the primary protection.
The "boob plate": is it an improvement?
For female armor, a common compromise between sex appeal and the desire to have armor that seems reasonably useful is to have an anatomical "breast plate" that closely follows the shape of the wearer's breasts, creating simulated Navel-Deep Neckline as a Distaff Counterpart to the male Greco-Roman muscular cuirass
. The problem with this is that if the cleavage is too deep, the wedge this creates would concentrate the blunt trauma of a blow to the armor right over wearer's sternum and heart instead of helping to dissapate it as a breastplate should, especially if the blow came from something like a poleaxe or a halberd, or another similar polearm, which is at least better than having a completely unarmored neckline but still potentially dangerous and far from ideal. Non-live action media enables the most extreme cases, in which a metal breastplate is depicted with such a skintight fit that in real life it would impair normal breathing. Of course, assuming that the armor is made from something like hardened steel, and gambeson is worn underneath - one would need plenty of good luck getting through all that, which is why going for the armpits, the groin, the back of the knees, or knocking the armored person over and shoving a dagger through an opened visor are far more viable options in any case.
Furthermore, the fact that most medieval and later European breastplates had a globose shape to make blows glance off — and thus created a fair amount of empty space over the chest — meant that little or no modification of the design was needed to accomodate a moderately-sized bosom. With that said, such a unisex design might not always be spacious enough for the more busty of female adventurers lacking the ability to bend the fabric of reality. In that case a version which pushes out the chest without the simulated cleavage (see Saber in Fate/stay night or Samus in Metroid for examples) can be a comfortable yet streamlined adaptation, and even a slightly more anatomical shape can be permitted as long as the design makes aesthetics secondary to safety rather than vice-versa.
