A320 Avionics ventilation
This is probably the most confusing thing about the A320 in my view - challenging the very essence of logical thought. It strikes me that very simply, you can boil it down to this...
If you press either the Blower or Extract buttons, you send the system to closed config (outside valves close up and circulation air is cooled against the skin), and AIR CON air is now used (in additon) to help cool the avionics.
That being the case (and at the risk of asking a stupid question), why isn't AIR CON air ALWAYS used, if available?