I actually think it's clever.
Very frequently, extra time is not as exciting as you'd think it would be as most teams prefer to play cautiously, hope for a goal on the counter but rely on penalties if that fails. To counteract this, rules such as the Golden Goal have been introduced, but that didn't have much impact - the incentive wasn't enough for teams to alter their defensive posture for extra time.
By introducing this away goal rule only after 120 minutes, essentially you take two teams that are level and put one ahead as soon as the extra time commences. So for that 30 minutes you now have 1 team that has no choice but to attack the game, which should make it more exciting. You also retain the benefit of incentivising away teams to score across the 180 minutes of normal time.
You could argue the rule doesn't seem 'fair' to the disadvantaged team, but I'd say it's still more fair than the away goal rule as it's normally applied - at least in this instance they get another chance to win rather than being immediately knocked out after 180 minutes.
All in all a sensible rule.
I guess this is what happens when you let the Football League decide the rules, and not the corrupt mongified twats who run FIFA/UEFA/The Premier League.