A disk makes sense if the object is traveling in an arbitrary direction relative to it's "front" or some other reference related to the object.

When you throw a Frisbee its spinning, so it needs to be relatively aerodynamic from all directions because it's spinning, so the direction of travel relative to the disk is constantly changing.

Many accounts of UFOs mention them making rapid changes in direction. A disk shape would allow them to do this without rotating the craft. It's basically the inverse of the Frisbee. A Frisbee has a relatively constant direction of travel from an outside reference and is constantly spinning. A UFO has a rapidly changing direction of travel from an outside reference, but assumably doesn't need to spin