One last thing, when you are using high contrast lighting (i.e. small light source relative to your subject), this WILL highlight wrinkles in your background, so a combination of blurring and softer light will help. The reason the direct light from behind the subject works, like SkipD mentioned, is because with a light so close and big, you get a more even and larger spread of light.
On a last note, I know we are all trying to get it right in-camera; however, if you are trying to get a light spot on the background to set off your subject from the background, but are getting too much light spread from a soft light source, you can always darken/vignette in PS. No it isn't the same, but at least you can get close. Sometimes we are working in spaces and don't have enough flags and reflectors to keep the light where we want it and need to use the digital darkroom to wrap things up in combination with our studio in-camera abilities. Larger areas provide more forgiveness in terms of apertures and subject/camera/background distance flexibility.
In addition to a more open aperture, if you can move your subject further from the background, or use a longer focal length, that will blur the background more.
The only problem with blurring is when you have a patterned background that is important to your entire objective for the shot.
One thing I ensured for the last living room studio shot of our daughter was to put weights at the bottom of the background (we didn't need the background to extend under her posing area) to keep it taut.
Just some thoughts.