Clothing is viewed as the first impression of a person.
Which person would an average individual find more trustworthy? A person with a collared shirt, clean shaven face, and an even haircut? Or a person with tatoos, ripped shirt saying "**** YOU," spiky dyed hair, and a cigarette?
Obviously, appearance is optional to the person. The cleancut guy could very well be a murderer, and the punkstar wannabe could be a shining example of human kindness. That is to say, appearance does not reflect your personality. However, strangers infer your personality by your appearance. A stranger would assume that the cleancut guy is alright, while they would be nervous around the punk. I could get a dozen tatoos, shave my head, get some chains, and walk down the street late at night. As a person, I have the right to dress as I choose and go where I please. However, people will avoid me. Police will be suspicious of me. Although my clothing is not representative of who I am, the fact that I chose to wear it causes other people to assume I want to represent that.
Now, onto the provocative clothing. In our society, it is presumed that less clothing means more flirty/open. Why? Because generally people only get naked with another person for swimming or for sex. Less clothing = more sexy. This should be wrong, but this is the societal view, perpetuated by both men and women. Because of this, it is decent advice not to dress half-naked at a party. By showing more of your skin, people assume that you are more open to flirting/getting it on.
That is the justification for the advice of "dressing decent." It seems to be fairly decent advice, just like "don't wear ripped clothes at an interview" and "don't wear clothing with racially offensive slogans." It is just advice, however. It does not have to be followed.
And this is the important line I feel. While it is good advice to dress more conservatively, you have the right to dress however you please. How you choose to dress should never be a case in legal rulings. And regardless of clothing, there is never a state of dress in which rape is justified.
Provocative clothing has no place in rape defense, as we have the freedom to dress as we choose. It's similar (not a great example) to how you can leave a window open at home. Someone may come in through the window and rob the place. But leaving the window open does not mean that the criminal was allowed to rob your house (nor should he get a decreased sentence). Likewise, I believe that it is alright to advise wearing more clothing, but a rapist cannot argue that her clothing was a factor. He is still just as guilty, regardless of what she was wearing.
TLDR: People make assumptions of certain clothing. Good advice is to dress appropriately, but you are allowed to do as you please. A rapist cannot argue clothing as justification for rape.