Robini there are a couple of fundamental reasons for the "retard" auto callout on the Airbus.
Firstly it comes down to the whole philosophy of the airbus FBW series. When we take off the thrust levers are moved from the idle (or slightly ahead of this if taxiing under power) into either the TOGA or FLX detents. TOGA gives you all the available thrust for take off although is not used that often. FLX is used in order to reduce engine wear. Once airborne and passing your thrust reduction altitude which is usually around 1500agl then the thrust levers are manually moved from TOGA or FLX back into the CLB detent. The autothrust will be active by now and will manage the required thrust settings based on pilot inputs. The important point to note here is that you DO NOT touch the thrust levers again until landing. The thrust levers also DO NOT move in flight, they remain in the same position unlike on boeing aircraft where they will be moving and provide a visual clue that some pilots like to see to get a "feel" for what the aircraft is doing. Therefore the retard callout is a reminder that you need to pull the thrust levers back to idle and more than likely select an amount of reverse thrust - can be idle or full as required.
Secondly, as has been well described if you do not take the thrust levers back to the idle detent then the autothrust will chase your speed and the autobrakes and ground spoilers will not deploy. This has been shown with tragic results in the recent TAM accident.
In summary if you are in a stablised approach then our FCOM's say to flare at 30' and the auto callout will come in at 20'. The flare is no different to flying a small aircraft with the pilot looking out the window far ahead down the runway judging the flare height, using your peripheral vision to give you a cue and using the autocallouts as further guidance. Of course autolands are different but I won't go into those as it will confuse matters.
Hope this helps.