Oh dear, I'm not sure about everyone's thought process as to how that would be flattering on every girl.
I used to be DD and these kinds of dresses and tops were the devil! It's very hard to provide the need support for the tatas within the bustier without straps. Also it brings quite a bit of attention to the twins. And some even make them appear larger--consequently ruining porportion. Since I'm already quite short with not much of a torso, I can't afford to do that.
But that's my experience with them and my one personal phobia against strapless things.
Perhaps someone else with a larger chest has had a different experience?