They aren't, as long as all you want is sex. But you might want a quality relationship, someone who makes you feel loved and supported, someone for the long term, a marriage that doesn't end with her taking half your stuff and leaving, or a mother of your children. Why would a promiscuous woman be suitable for these roles?
It has always been more useful to assess a person by what they do than by what they say. What is a promiscuous woman's behavior telling you? It is telling you that she lives in the moment and will act for her immediate gratification. It is telling you that she is ok with discarding relationships close enough to be sexual ones. It is telling you that she is often open to the next exciting thrill that comes along and thus might be about to cheat on you or leave you for the better looking guy at the gym (and there is always going to be a better looking guy at the gym.) It is telling you that she rejects some of the long held societal norms of female behavior.
Of course all that is totally her right. It is her life and her body. She can do whatever she wants with it. I am not in the least bothered when women decide to take this path. But why should a man who wants more than sex want to have a committed relationship with this person?