While CDA is often great , sometimes it is not practical when you consider there are a hundred airplanes sharing the airspace....sometimes separation from other traffic is more important than saving 6 dollars in fuel.
I recently saw someone do 300 fpm descent into JFK....Atc said I need your best rate to 4000... Then said expedite to 4000.... So they went to 400 fpm.
The 3rd call from Atc wasn't so pleasant!
If you get violated by Atc and are invited for tea and biscuits.....will your company back you up cause you saved few bucks?