I know there are a quite a few photographers who believe using natural light is the only way to shoot. And while I agree that stunning portraits can be had using natural light alone, a little fill light from flash or a reflector can control where shadows fall and can enhance rather then detract from your outdoor portraits.
The basis of the medium of photography is light. With that in mind the more light you can bring to your portraits, and the more you can control that light, the better your portraits will be. The main benefit of using additional light (flashes, reflectors) is to control the shadows and stray light that can cause distracting hot spots and shadows to fall on your subject. The benefit of shooting with additional light is to highlight the subjects eyes and hair. There is a vibrancy that is lost if additional light is not added to the portrait. The hair goes flat and the eyes look dark and without life. How many times have you heard someone comment on a photo by saying "There eyes are so bright" especially when commenting on a photo of a child. It's not the eyes that are bright but rather the reflection of light in the eyes. The added light makes them come alive.
The kind of light you use, reflected, strobe, or flash gun is not as important as it is to just simply use the additional light to your advantage and to the advantage of your subject.
The above photos are an example of using flash outdoors. On the right the models eyes are dark and her hair looks muddy and without texture. On the left her eyes are alive and her hair shows all the wonderful texture of each curl and wave. Both photos were taken at the same location, time of day and in sequence. I exaggerated the amount of light to show the difference in detail. A softer broader light would still give the feel of natural light and can be achieved by using a diffuser.
So pack your flash, invest in a reflector (the bigger the better) and pray for open shade (the best time to shoot outdoors is an overcast day which gives broad even light, or under the umbrella of a large tree). And never forget that light is the currency of success for your outdoor portraits.
