If they had approached the target from above the trench, they would have been exposed to the surface guns and torn to bits. That was the whole point of the trench run: It was a daring technique made necessary by the nature of the Death Star.
If you can approach the death star from any location into the trench then why can't you approach it at the part of the trench were the exhaust is? Besides the laser towers in the trench can't hit the x-wings coming right for them so approaching a turret from above or from the side doesn't really matter.
It was a daring technique necessary to make a cool movie. I love the trench run and it's way cooler than the podrace.