An inside out paraboloid would theoretically produce isentropic compression for an inlet cone. But this question is about nose and tail cones on fuselages, and leading and trailing edges of wings.
An isentropic nose would preserve pressure in the region surrounding the aircraft, so that more of it may be recovered at the tail. This effect, if course, may not be very significant. It would also reduce aerodynamic heating.
An isentropic tail would enable better pressure recovery following the expansion fab, and thus reduce drag.
So, why are these shapes not used?