We are all free to wear what we want, sure. But why wear something that doesn't look nice, when better looking options exist? Of course there are plenty of people out there who go out in pajamas or sweats and obviously don't care at all how they look, so obviously not everyone is going to agree on this subject, lol!
Here is my take on what makes an outfit appropriate, or not: clothes, just like body language, tattoos, etc, are a form of non-verbal communication; what a person should wear depends on what they want to communicate. That's why it is appropriate (just for example) to wear a sexy outfit on a date or to the club; if you are trying to attract a mate (or if you just want to fit in with all the other ladies who are trying to attract dates), a sexy look is totally appropriate. Wearing the same outfit to the office would be inappropriate, because you (generally) don't want your coworkers to think of you in a sexual way. To take another (non-sexist) example, think of a guy in full leather motorcycle gear; if he is with his motorcycle club, he is wearing appropriate gear (leather is safer for a bike, makes you look tough around the other men, etc); however, this would be inappropriate for the office because, at the office, you are probably not trying to communicate a tough-guy image. Just like a woman at the office is (hopefully) not trying to communicate a "sexy" image. That's how I look at it, anyway.