Well like RPG characters & H-heroines wearing only steel chest plates, and depending on the quality of the shells themselves, they will most likely chaf the poor female's skin on "that area". Specially with swift or abrupt movements (especially the "healthy" & bouncy ones).
Isn't it also tradition to not wear anything under a kimono?
It's more of a healthy habit. Woman with big breasts tend to feel 'constricted' in their sleep if they wore bra, even the support ones. And unlike what most peoples say, bras DON'T prevent sagging. More info here: http://www.womenshealthmag.com/health/bra-safety