I prefer to use available light for my twilight images. I feel working with available light captures the romanticized ambiance and character of the project with a degree of authenticity that cannot be matched when using supplemental lighting. The results are always stunning.
I can create only one dusk or dawn image per day because this technique requires precise timing. At sunset or sunrise, normally there is only a two minute window of opportunity when the light levels are balanced just right and we need to be prepared to capture the image at that precise moment. We do not have the opportunity to move the camera to a second position for an alternative view.