I believe it had to do with rapid decompression profile...originally you were able to get full speedbrakes with ap on with the 320. However during a rapid decompression senario when you deployed the spoilers at altitude coupled with the slow pitch over, the wings were loaded up and it would trigger a high aoa, retract the spoilers and produce a spoiler disagree message. So you would either have to reset the spoilers, or more than likely you would not notice them and trying to get down in a hurry without the spoilers. Additionally I believe that the full spoilers with ap on with the 319 is the same as half spoilers with ap on with the 320.