Maybe the zombies have limited climbing ability or there's a special that crawls along walls and shoots projectiles at the player.
Zombies can climb, actually, but I can't think of a processor-friendly way to get the AI to climb, unless I just hack it up and make the zombies levitate if they're next to a wall they could theoretically climb.
As for crawling along the walls, eh, I just don't see that happening. A spitting zombie would be nifty, though.