[VivaLFuego]

Its more practical for operations that can satisfactorily adjust capacity purely on frequency (which includes most metros outside the US). When you're cutting and combining train consists frequently between time periods, you'll add a lot of extra yard labor that a standard coupling mechanism otherwise handles. WMATA, like CTA, operates a range of 4/6/8 car trains on different lines at different times.

Also, I forget where, but I've definitely seen at least one study that found the increase in capacity was almost negligible, since in crowded conditions people still tried to stay near the doors. The end of a car or an articulated joint are equally undesirable.