I like to work with only one light during certain photo shoots, especially on-location and outdoor. Having only one light in such situations (in my case it's usually the SB-900 speedlight from Nikon) gives me lots of freedom in terms of movement from one location to the other and in terms of space. When I shoot with only one speedlight I also use an umbrella (almost 100%) and of course a light-stand. I used exactly that setup for this quick Forest Nymph shoot.
This compact and portable setup is quite small, light-weight and allows me to set it up almost everywhere because it doesn't require much space - a perfect situation for every photographer and that's why I love it. Ohh and one more advantage of using such simple setup is that I can concentrate on the subject and interact better with a person and a client instead of spending too much time on setting up the equipment and worrying too much whether or not everything is set up as it should be - this is something I've learned long time ago from Bert Stephani.
But of course, there are some shoots where I need more light, more light modificators, more equipment, and much more muscles to carry everything around :)