Nomu doesn't seem to be a totally successful genetic engineering product. Something went wrong and left his brain exposed. Even if he avoids physical damage to the bare brain, it's much harder to avoid infections. Not to mention he still has a funny danger of having his brain drop out of the skull cup during high deceleration, such as if he's sent flying backwards and then hits a wall. The brain might just continue flying while the body stops.

But yeah, that was a jolly good part, Shinta. I suppose one could say as expected from a girl who willingly chose such an attire to begin with, when for example Nice Person was lamenting her (awesome) choice.