Also, limb wounds and such might not kill a walker instantly, but they still have blood, they still have internal organs that require certain processes to work. Just because walkers are governed by pure instinct doesn't make their biology any different from ours. They'll bleed out just like any other human.
In the first episode we saw a "walker" that was basically a half eaten torso, complete with a full exposed rib cage and the thing was still going, so I don't think bleeding out is an issue.
Yeah, trying to use real medical and biological rules for zombies quickly becomes a losing proposition. WW-Z tried mitigate this a bit - the black ooze/blood that is toxic to all other life, including other microbes and parasites, and seems to keep them going even in deep ocean pressures - but the more you know, the harder it is to buy a "scientific" explanation.
The problem with that World War Z excerpt is pretty straightforward:
Slow zombies can't climb. So they have absolutely no way around or over any obstacle the height of a man or taller.
Slow zombies also don't have the brain power to detect traps.
You set up tall barriers, you lure the zombies up against those barriers by waving at them or what have you, and you kill them. All day. Or once they're stacked up against your barrier a kilometer deep, you helicopter out and napalm the site.
An enemy that can't hide, can't retreat, can't stop himself from advancing, and can't overcome simple obstacles isn't really an enemy. He's an annoyance.
Brooks made that battle work by having the military be too dumb to set up a simple stone wall 8 feet high in front of the advancing zombies. Or if there was no time for stone, city busses lined up nose-to-tail.
I think a key point is that they didn't know any of those things at the time; as somebody else said, it was early in the war, and Brooks devotes a lot of time to all the bad information floating around. Ironically, the same thing that makes zombies biologically implausible is what makes disastrous counter-strategies more likely; the authorities simply can't/won't accept that these things are reanimated corpses that can only be stopped in a very specific way. It's just "African Rabies", or whatever.
Having said that, I do agree that really, you would need mass infection throughout the population, then the zombie stage later, in order for this to get really out of hand. Especially if people reanimate automatically on death, and it wasn't known at first. Hospitals might be the first places to get overrun, actually.