Street photography has been around almost as long as cameras have. Street photographers capture the true feeling and spirit of places and people through this documentary style of photography. Public places are free game to photographers and they do not have to have permission to take someone's picture. On top of that, the photographer owns the rights to these images. Done right, these images are brilliant in their artistry and I am a huge fan.
Just like street photographers, portrait photographers own the legal copyright to the images that they produce and that photographer can use those images for whatever purpose he sees fit in order to help his business. He might use the images for marketing brochures, advertising, booths, shows, contests, etc. A photographer is an artist and he owns the rights to the art that he produces. It's as simple as that.
I am all for art and I totally agree that the images produced by photographers should belong to the photographer. That artist's blood, sweat and tears go into those images and you have no idea how many hours are spent on a single photo session. A photographer is extremely picky about so many aspects of an image starting from the first camera setting to the placement and opacity of their watermark when revealing those images to the client. They want to make sure that they offer their clients something that they will cherish and be proud to display; at the same time, they are creating for their clients a piece of art that represents their own personal style and artistry.
This is one reason why I think it is only right to ask permission of a client before using a photo from their session for promotional purposes. None of us sees ourself the way others do. My feeling is that if the person in the photo is not comfortable with everyone seeing it, then it has no business being shown to anyone else. Images taken by a photographer for a client are for the client first and foremost. If that client is comfortable with the photographer using their image for his business then that's a bonus for the photographer.
If a photographer asks you if he or she can use one of your session images, it's because she is very proud of it and thinks it represents her business perfectly. (Remember, photographers are picky people). If you are not comfortable with your picture being out there, then just say so. If you are, or even if you are on the fence, trust your photographer when she thinks that others will see your photo as the beautiful image that it is.