I recently did an exam on this, for what that's worth. These are the boxes I was taught to cross.
ATC objectives (as per ICAO Annex 11) definitely include avoidance of collisions between aircraft, and between aircraft and obstacles, but also the orderly flow of air traffic.
It ATC become aware of excessive traffic, they should notify other units and the PIC's affected of "excessive delay".
I guess non-PPR traffic could be advised of delays exceeding their fuel reserves, or more simply told they will not get a clearance.
Perhaps one of your ATC friends will pop up shortly with the official version.
Last edited by 24Carrot; 1st Mar 2011 at 09:46.
Reason: Removed the wrong smilie