My reasoning about the whole running zombies is this...
Back in the 1970's the thought of the dead rising from the grave to feed on the flesh of the living was enough to scare the shit out of people.
But, nowadays all the violence we see on tv everyday has cast an imunity over our minds. The regular things that would have scared us silly thirty years ago don't quite have the same effect.
So in this remake of a great classic they had to pull something out of their asses to scare us. I believe that the screen writers for the remake of Dawn of the Dead payed a little too much attention to 28 Days Later in which the "infected" could run. So seeing in screen tests (The idea that the zombies or infected could run) that the audience jumped at the parts where the heros were being chased around by fast moving enemies I'm sure played a big part in their quest to scare moviegoers.
I mean would you really have been scared in Dawn of the Dead if the zombies couldn't run? I know that I wouldn't, the thought of the massive army of zombies that are capable of running were outside a mall I took refuge in....I would have shit my pants!!