The away goal rule was often unfair.
UCL Semi-finals 2003:
AC Milan vs. Inter Milan
First Leg: AC Milan 0–0 Inter Milan
Second Leg: Inter Milan 1–1 AC Milan
Same venue, only one week apart. Milan went through on away goals, which obviously didn't make any sense.
Not to mention that some managers like Mourinho and Simeone built their European careers on scoring the minimum of away goals and go through on aggregate draws while displaying horrifically boring defensive strategies.
Lask Linz vs. Celtic this week was a good example of why the game is more fun (for the neutrals especially) without the away goals rule. Celtic scored first, and at that point Linz needed 4 goals to equalize on aggregate. With the away goal rule, they would have needed 5 to qualify and no chance of a draw + penalties. This small change, motivated them to press and actually score 5 goals, without worrying that if they conceded one they would have to score an extra 2.