A shallow depth of field is something that portrait photographers use quite often to separate their subject from the backdrop.
Click to view larger. Canon 5D Mark II, Canon EF 50mm f/1.2L at f1.2 (wide open).
When the subject is all in the same plane, it is most-definitely advantageous to shoot this lens wide open. It really helped with this photograph to have an aperture that large (for more about apertures click here) for this shot because the model was standing, just like me. If we were lines, we would be parallel. But what if our lines bisect? Is this aperture still the best choice? Sometimes. Sometimes not.
Click to view larger. Canon Rebel xTi, Canon EF 85mm f/1.2L at f/1.8 (almost wide open).
I stopped this down when I took it - intentionally shooting at f/1.8 instead of f/1.2 because of the way I was below him slightly. Had I shot fully wide open, the background would have been blurred, but his shirt would have also been blurred. I'm not saying it is wrong to shoot like that, but in this case it was not the look I wanted, nor was it the look the stylist wanted. So I needed an aperture that allowed both his face and his costume to be inside the depth of field. Generally, if a photographer chooses to keep only the model's face within the depth of field, it is a make-up advertisement or a head shot for the model. It can also be used as a tool to hide physical attributes about the model's body that they do not like. However, in fashion photography, it is generally frowned upon because the designers want to see their clothing in focus.
Click to view larger. Canon EOS 5D Mark II with Canon EF 50mm f/1.2L at f/1.2 (wide open).
This is an example where it would have been a good idea to use a bit smaller aperture. Because of my location below the model looking up, his face and eyes are not in the depth of field. Sometimes this works, and this photo was actually what they were looking for, but I would have liked to have his face in the depth of field plane.
Click to view larger. Canon EOS 5D Mark II, Canon EF 50mm f/1.8 at f/22
As a quick note about studio photography... when you are shooting with a controlled backdrop, it doesn't matter as much what aperture you use because you are not hiding anything in the background. When picking an aperture for the studio, it is best to pick one that has the whole model and wardrobe inside the depth of field and is appropriate for the lighting you are using. Choosing a middle-range aperture is fine if you are not using a lot of lighting because it will keep a reasonable shutter speed instead of a long one.
Photography is an art form, one which should have no "rules" so I am not saying that having part of a subject outside the depth of field is something that should never be done. However, generally it is not something that a client wants from a portrait photographer.