I never understood this... at least it added SOMETHING random and unknown to the combat, rather than "sigh, another 3 guys to kill... click click click moving on". Having to make and change plans in the middle of combat was more tactful in DA2 than in DA:O because it was WAY easier to control the fights in DA:O.
It was not "fail" unless after playing the game for 3 hours you STILL didn't start planning for the second or third wave of guys...
There were lots of problems with DA2, spawning waves of enemies like they did could easily have been overlooked if the rest of the game was up to snuff.