A shambling mass of brainless human flesh motivated purely by instinct? I'm sorry, that threat is insignificant compared to fighting actual human beings.
That's one reason I kinda preferred the super-fast zombies of 21 Days Later and the Dawn of the Dead remake. It made it a bit more plausible that those things could overrun the military and wipe out the world's population in such a short time I thought.
It'd make a slight difference, but that's about it. The whole thing would still be stamped out pretty quickly. It certainly wouldn't lead to the collapse of civilization.
Biting and scratching is simply an ineffective way to become a pandemic. Otherwise, rabies would be a much bigger problem than it is. It's worth a chuckle that several attempts to make zombies more plausible have involved mutated strains of rabies. Why writers think rabies would have a different effect on a human population than it does on animal ones is beyond me.
But again, as soon as you make it airborne, the idea loses something. You end up with Outbreak
instead of Dawn of the Dead.
Sometimes the attempt to make this stuff more plausible works, especially if you find a way to tie it into the point of your story. But the writer has to realize when those details don't matter. Otherwise, you end up with midichlorians.