I just can't see a decomposing corpse sprinting. To me, it makes it cheesy. The slowness is half the scary of zombies. It's like Mike Meyers. He never runs, just walks, yet he's always right behind you. That is intensely creepy.
When the zombie runs, it just looks silly. Like I said, it's breaking the rules for a cheap effect that, I think, really takes away from what makes zombies special. And while I do understand that a new zombie that hasn't decomposed much might possibly run, there's still things like rigor mortis to consider. Taking the stiffness of death away from zombies makes them less like the walking dead. That's what I loved about the Romero movies, it made us face the reality of death. Sprinting zombies strikes me of more a fantasy version of what death is, it puts a filter between me and death so that I am no longer scared by mortality, but rather by cheap creep-tactics.