You and your 4 Battle Brothers are part of a squad of 30 men. The 'fog of war' is heavy and just to your right you see another 5 of your battle brothers slowly being swallowed by a gush of the fog as they proceed further down the line. All of a sudden your sensors pick up several enemy moving quickly behind you. In another heartbeat bolter shots and battle cries are heard. Suddenly over the vox you hear "We are undone!" "Defend the line!"
What do you do?
1. Turn your back on the enemy risking a shot in the back and run into the fog not knowing what you will bump into and what you may kill or be killed by?
2. Hunker down and shoot the enemy you can see? (This is representative of a Fire Fight)
3. Charge (counter-charging) the enemy (that you see) and bring glory to the Emperor by slaying each of the infidels with your chainsword? (This is representative of your Close Combat Attacks)
4. Drop, cry and plead to the gods as you are scared shitless (A real life situation that is not represented in the game as part of an initial assault)
Options 2 and 3 represent the countercharge that happens in Epic
Option 1 represents turning your back on an enemy (with no regard to cover) which would otherwise have you killed.
While looking at the battle from a 'God' perspective, you have to remember that each base is representative of 3-5 men. With enemy in sight, the majority who counter-charge will not turn their back on the enemy they can see - regardless of what you know from a 'God' view. Those tanks have hatches. Each marine carries Krak grenades and can force open hatches. Even basic troopers have ways of dealing with cumbersome tanks - my favourite example is one man (Indiana) shoving a rock into the barrell of a gun for it to backfire

So it is representative of the fog of war and it is not a silly or broken rule. It is a Tactically Sound rule that if exploited by a general, allows them to have more control over an assault by splitting the forces of the enemy. This has been shown time and again in many great battles of history (feints techniques and splitting the enemy frontage) to eventually win a battle.
When my forces are the victim of it, I congratulate the opposing general on his tactical stratagem rather than ruin the moment with a perceived opinion on how the rule is broken. It makes for a better game and you learn from your experiences.