Imagine that on Tuesday night, rather than playing that late free-kick short so Milan lost possession and conceded a fourth goal, Robinho had hurled it into the box. Imagine Philippe Mexès had jumped for it, the ball had taken the merest brush off his pony-tail and that had been enough to take it past Victor Valdés. That would have made it 3-1 on the night, 3-3 on aggregate and Milan would have beaten Barcelona on the away goals rule.

Except they wouldn't really have beaten them, would they? They'd have gone through by an arbitrary regulation so familiar that we tend just to accept it. Or take the other game: at 0-0 Schalke were going through on away goals, at 2-2 Galatasaray were; two level scores, two different outcomes. In the end, of course, Umut Bulut's goal gave Galatasaray a 4-3 aggregate win – but his goal came about because Schalke had committed players upfield as they knew they were going out on away goals.

Or take Wednesday night: if Arsenal beat Bayern Munich 2-0, do they really not even deserve extra-time? If Málaga beat Porto 2-1, do they deserve to go out? Or on Thursday, in the Europa League: Newcastle defended superbly to draw 0-0 away to Anzhi in the first leg; if a defensive slip-up costs them an early goal, why should they have to score two to avoid going out? Why does it matter whether the slip-up came in the fourth-minute of injury-time in Moscow or the first minute at St James'?

This is the first problem with the away goals rule: it simply isn't fair. It makes certain goals count for more than others. When M'Baye Niang's shot hit the post at Camp Nou he was effectively denied not a goal but a goal and a half. If that generated good football, made games more exciting, then perhaps the inherent illogicity of the rule could be tolerated. But it doesn't. In fact, it achieves precisely the opposite of what it set out to do.

The away goals rule first made an appearance in European football in the Cup Winners' Cup in 1965, primarily to eliminate the need for replays, which were costly and difficult to arrange. Given the alternative was flipping a coin, it probably seemed the lesser of two evils and, besides, back then it made a certain sense. Only 16% of all European away games then resulted in an away win. Away trips were difficult: travel was gruelling and away teams would often face unfamiliar and/or hostile conditions. As a consequence, the tendency was for the away side to bed in, look to absorb pressure and try to keep the score down. In the 1964-65 European Cup, for instance, three of the 30 ties featured leads of two goals or more being overturned.

A 2-0 deficit was seen as eminently recoverable. What the away goals rule did was to try to persuade teams that a 3-1 defeat was better than a 2-0 defeat, to encourage at least an element of risk-taking.

But circumstances have changed. In each of the last five years, between 30 and 35% and matches in European competition have been won by the away side: even if you wanted to make the argument that the away goals rule has worked, the original rationale for its introduction has gone.

"In competitions where conditions, home and away, vary greatly — in, say, the African Champions League — away wins remain very hard to come by," Ian Hawkey wrote in Issue Zero of The Blizzard. "Poor, or fearful, refereeing would count as a factor in Africa. So would vastly distinct standards of playing surface, or the fact that a pair of matches in two-legged tie might easily take place in different seasons: winter in Tunis is scorching summer in Cape Town. In those circumstances, the away goals rule clearly has an important compensatory value. But in the European Champions League, it scarcely does. Where the European Cup of the 1960s and 1970s was exotic, with a greater range of destinations and opponents, the modern format is repetitive, cliquey."

Transport is better now, there is a great homogeneity of conditions while the differences between a German side and a Spanish idea, say, or a Russian side and a French side, are far less than they were.

Teams are cosmopolitan, national styles less distinct than they once were. Away trips simply aren't as frightening as they once were and so the away goal becomes a weird distorter.

To see how this can spoil football, you only have to look at last season's Champions League semi-final between Bayern Munich and Real Madrid. Bayern led 2-1 from the first leg. Real Madrid, seeking an early equaliser at the Bernabéu started furiously and struck twice in the first 14 minutes. That forced Bayern on to the attack and they made it 3-3 on aggregate with a 27th-minute penalty. It had been a thrillingly hectic opening half hour, and then the game died. Bayern looked to protect a level position on away soil, while Real were inhibited, knowing that if they conceded one they would have to score two to stay in the tie. Bayern behaved just as any away side would; Real were forced on the defensive by the away goals rule: far from encouraging the away team to play progressively, the away goals rule encouraged the home team to play more cagily.

It's a stance widely taken among coaches. "I believe the tactical weight of the away goal has become too important," Arsène Wenger said at a conference in 2008. "Teams get a 0-0 draw at home and they're happy. Instead of having a positive effect it has been pushed too far tactically in the modern game. It has the opposite effect than it was supposed to have at the start. It favours defending well when you play at home."

So the regulation is unfair, it's illogical and often achieves the opposite of what it's supposed to do. Why do we still put up with it?