Analysis / Chainmail Bikini

This trope in real life

Instances of this trope in real life occur only 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 whenever one of them was wounded.

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 Absolute Cleavage 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, 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.

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.
http://tvtropes.org/pmwiki/pmwiki.php/Analysis/ChainmailBikini