I believe that when lighting with raw tungsten lights, and the main light is pointed at the subject, you use a fresnel. Any other lights such as rim lights and fill lights you can use barndoors etc. I think that if you don't use a fresnel you will see the paterning that is visible from the bare tungsten tube.

The purpose of the fresnel lens is to collect the light rays and send them in a directed or focused manner therefore giving you the hard edge you are looking for.

Michael McBlane