Many of these films are rightfully dismissed simply as bad attempts to make a genuinely serious apocalyptic narrative. Whether it's due to filming, acting, or plot, there really isn't much to redeem them beyond a certain "so bad it's good" entertainment value. One of countless examples would be John Carpenter's 2001 masterpiece, Ghosts of Mars, starring Ice Cube and Jason Stathan, among others. Or just about anything that shows up on 'fearnet'. I want to stress that I absolutely love many of these movies, they just don't represent what I'm trying to get at.
However, I think there is a subset of B-horror monsters that have genuinely defensible, if campy, value. Whether it's superhero, combat or horror movies, It seems to me that there is a strong realist/extrapolation trend in most genres today. The B-creatures I will talk about go to the other extreme, overstating chaos, violence and frequently gore to an almost surreal extent. Any movie like this walks a fine line, but for me one of the best examples for zombies is Planet Terror by Robert Rodriguez. Some other not-strictly-zombie examples would be the Evil Dead series, Starship Troopers or (maybe closer to just 'bad', but I'd stand by it) Jeepers Creepers. These movies have plenty of satirical/and humorous elements, but they aren't really "parodies". In Planet Terror we have plenty of zombie tropes, from encountering the first few walking dead, to emergency rooms being over-run to desperate last stand. But what sets its apart from being a standard satire is the uniquely outlandish turns the plot takes, and the cartoonish style of it's violence. *Spoiler, such as it is, in white text*: There is no zombie cliche' for a standoff between survivors and the partially zombified remnants of a special forces team that killed Osama bin Laden, or Ash Williams chainsawing his way through a medieval undead horde.
It seems to me that stories like these take a kind of expressionist view of horror, saying much more about how we depict evil or how we characterize our protagonists/villians than any particular metaphor or statement about the human spirit. If stories like World War Z are an extrapolation of our world, these types of narratives seem to me like extrapolations of the horror/sci-fi genre as a whole.
...in fairness, there's also probably a level at which many of these stories are making a point about taking anything too seriously, so here's some hyper-dramatized, irreverent B-horror in song form:
http://www.youtube.com/watch?v=WzouD2Ssg8s