CDAs are a simple idea, complicated by ATC demands and misinformation: a surprise shortened 25 track miles, instigating a dirty dive, regularly lengthens to become 40 track miles and 200 fpm V/S... Gotta love STN...
One has to use all descent modes (B737) to ensure you can adapt to whatever is thrown at you. If that involves a period at -500 fpm, fair enough.
As for fuel burn, of course a continuous descent from cruise to flaps will be most cost effective, but this is rarely possible in a busy TMA. I'd guess this is seen as one of the operating costs, similar to the many kgs we burn while holding off from the stand waiting for our useless ground crew to turn up...