As usual, a lot of rubbish spoken here.
According to Airbus (if anyone could be bothered to read the document), the 319 saves 1 Kg of fuel per knot reduced in the descent (260kt to 300kt range).
If you perform an intermediate level off, the descent speed may well drop from 275 ish to 255 ish.
The SOP allows speed to be specified by ATC, or indeed the commander may choose a suitable speed for the ATC environment. Sounds like some Captains are not doing either!