Because the ground speed mini in rough conditions could change a lot in a very short period of time so for me is better to have selected the Vapp (blue) and adjust the thrust as in a Boeing class aircraft.
The whole point of the Airbus ground speed mini system is to react to gusts and keep the wing at a safe speed above the stall, while also keeping a constant groundspeed. Airbus fly constant GROUNDspeed on approach with a suitable margin above the stall, Boeing fly a constant AIRspeed only, (I don't know about 777 or NG).
Going to selected speed in an Airbus in such conditions switches off the groundspeed mini system and could be incredibly dangerous.
One should never override systems simply to make your flying task easier - especially if you don't fully understand or appreciate how they operate. The ground speed mini system was invented for a very good reason.
U