SATC question

Went to see "Sex and the City" movie last night with some of my gal pals. Ok, the movie was good, just like the t.v. show, you know, you laugh, you cry, the whole deal. So here is my question? Why does Carrie think it is OK to wear a gorgeous (or not so gorgeous) dress with the back cut out with a black (or other color) bra? Did I miss something in the fashion world that said it was OK to have your undergarments just totally out there? I don't get it. I have always wondered about this even when the show was airing originally. She looks so great and then she turns around and blah- like it is screaming in your face! I mean with the popularity of thongs, seamless undies, and convertible bras and for heavens sake, stick on bras, can the girl not get it together?! I just don't get it...