That is a good question... Perhaps it is what that piece of clothing represents, like most things we wear.
All of us are born naked, yet we feel embarrassed to not have such a flimsy layer of weaved cloth covering parts of ourselves when in public, or even in private.
With one able to see Rarity's or any other woman's panties, the more perverted-minded think of what lies behind it, while those with respect towards her look away from such areas. By keeping her covered; the societal norm of clothing is kept, while still keeping the mystery of what she truly looks like underneath a vaguely defined outline, leaving it up to the imagination of those looking at the piece.
I don't know perhaps Im just prattling on, never mind what I said.