Full speed brake is available on the A319 with autopilot engaged however the panels only move to half of the position that they would on the A320.
Therefore an A320 with A/P engaged and half or full speedbrake selected = A319 with full speed brake.
We fly both types and the A319 doesn't slow down as quickly as the A320 on approach, some pilots need reminding that full speed brake is available with the A/P on, the halfway position has very ltttle affect.
Speed brakes may be used in DES mode, indeed the aircraft may call for MORE DRAG, typically when descent has been delayed and a speed or altitude constraint needs to be met on the way down. Needs to be watched carefully or you could be flying with brakes out and power on once the constraint has passed and you are back to profile.
I have been told that engine anti ice used on the A330 in descent may require the brakes due to the bump up in idle speed.