Combat in AW is pretty much based on what you want to accomplish, not just inflicting harm for harm's sake. Usually, you're fighting a person with goals that conflict with yours, so the Aggro options reflect that. Seize By Force is a little more "let's all just hurt each other," so people often go to it first as the obvious "rpg combat" choice. Seize works just fine if the character is trying to get something from the zombie horde, keep something from them, or run out in their midst, flailing swords like a crazy woman.
(Edit: I wrote up some stuff about a custom move, but changed my mind. This is what I wanna say instead:)
The more I think about it, though, the more I'm kinda like "either they're out there just fighting or they're not," so Seize often works fine. What are you really fighting zombies for? To clear the safehouse? Then you're 'securing your hold,' that's a Seize. Are you just trying to kill a bunch of walkers like, just to do it? That's probably dumb of you, but that's basically just Seize as well.
Basically, if you eliminate Aggro for zombies (because you cannot threaten it or change its behavior), that leaves Seize. And Seize is inherently dangerous and risky... which is maybe exactly what you want fighting zombies to be. If you want to stay 100% un-bitten by zombies, don't go out and fight them, right?
Aggro is good for going all sniper on a dude, but who really cares if you shoot a single zombie? It has no friends, no relationships we care about. If the player gets out her rifle, I'd probably just go "sure, great, its head explodes" and save the dice for risky entanglements, handled by Seize and Under Fire. Makes sense to me.