MF - It wasn't a personal critism of you or your experiance, so if an apologee is required then please humbly accept it.
Airbus specifically state that the IDLE factors is an operational tool used to adjust the assumptions of the descent path computation and is NOT a tool to save fuel. Improper use might lead to inaccuracy of time and fuel predictions as well as the flight path computation.
Yes you would go below the profile, but in order to re-capture the ideal profile, whilst remaining at idle, one would need to reduce the speed; something managed DES does for you with the use PFD speed brackets/ ATHR combination. Additionally the fuel flow values at idle are not that accurate due to design and sensor location.
LYKA