Fast zombies aren't interesting because they lack subtlety. They run without tiring and sprint maniacally towards the nearest living creature. Traditional zombies, with their lurching, intermittent style of movement are somehow more realistic. They imply that the conversion to undead status is not merely an addition of cannibalistic hunger but a subtraction of vital energy. In the real world, even the fiercest beasts have weaknesses, have their moments of rest and hesitation. Zombies aren't real, but then neither are alien monsters, demons, vampires, and the like. All these fantastic creatures are better depicted with that drop of realism, with that ounce of vulnerability.