I have recently started using a single flash during some of my client photo sessions. The use of the single light is very useful for that touch of fill light that you need to brighten certain areas or features taken in shaded areas.
One common misconception is that you need more than one flash unit to get any decent light with off-camera flash. While having additional flash units does help, there are an incredible amount of things you can do with only one. Using some simple workarounds you can often make it look like there are more then one. I am a bigger fan of only using natural light during my sessions, but on one occasion, the client needed photos quickly and time did not allow for a day light session.