This might be one of those personal preference things, but I like to use a flash even when I’m taking photographs outside in broad daylight. The reason is twofold: First, it fills in unflattering / harsh shadows caused by the sun; and second, it really brings out the eyes. Last March I spoke with students at DDSO (Developmental Disabilities Service Organization) and I took two real quick photographs to show them the difference in the eyes.
As you can see from this example (click for larger photo), the flash nicely brings out the eyes. It fills in the shadows caused by the brow and nose, while also adding a beautiful catch light that I really think helps to make a picture pop.