Youre asking the wrong person, because I enjoy my nips showing :) I think its very attractive and natural. Even when I wore bras I only liked the unlined ones ;)
I think its an American thing to be so worried about nipples - as if there is something wrong with having them.
They say because it distracts men in the work place - well, I find errect mens nips pleasantly distracting too sometimes (if they guy is hot) :)