Use a sedan instead of a conventional two-door coupe when regular rear-seat access, passenger accommodation, and mixed everyday errands matter more than the coupe's particular styling or driving appeal. Separate rear doors can make frequent boarding and child-seat access easier, while a suitable sedan can carry the normal passengers and their luggage without repeatedly moving a front seat.
Compare the actual cars. Sedan and coupe names are not perfectly consistent, and some models marketed as four-door coupes already provide separate rear access. Door count also does not guarantee headroom, cargo capacity, comfort, safety, or cost. Choose the sedan when its specific layout solves the owner's recurring needs better.