First of all, the "zombies" as in the rotting corpse type are not possible on a mass scale. Once the brain ceases to function, the cycle of sending electrical impulses to beat the heart and send blood through the body, and feeding the brain with oxygen, breaks down and cannot be restarted just out of the blue. If they're dead, they're dead for a reason. And if the dead did walk, what about rigor mortis? Zombie isn't really scary when he's petrified, now is he? And how would their muscles continue to function if they move and tear them without any biological processes to regenerate the muscle tissue? They would atrophy very quickly, and their jaw muscles probably wouldn't be strong enough to penetrate flesh after a week or so.
How would a zombie "plague" even commence? We've all seen enough movies that if we see a guy with glazed eyes ripping out some poor bloke's throat, we know something is not right. And I don't think someone will just stand there gawking when all attempts to reason with the violent individual are in vain. I'm sure that in Arizona there are more than enough gun nuts to quell any scale of invasion before it even begins.
And if for some reason zombies did infest a major city and were out of control, the government would wipe that place off the map without a second thought. So there's no heroics, no guerrilla warfare against the horde with your katana or whatnot; just a daisy cutter or MOAB vaporizing you and the walking corpses.
I personally like the idea of zombies, and I am a fan of the literature involving them, however the likelihood of an actual zombie outbreak is astronomically small. Is a "zombie" even possible? Show me the science of how you can reanimate corpses with a bacteria or virus and maybe I'll concede that.
