In fiction, zombies are a metaphor: the soulless, soul-sucking horde that will never stop coming and will drag you down eventually, no matter how much ammunition you're carrying. And they'll turn you into one of them when that happens. The metaphor usually keys into xenophobic fears, such as plain old-fashioned racism (zombies offer a politically acceptable alternative to ranting about the asiatic hordes outbreeding the white master race and eventually diluting them into mongrelism and extinction), or fear of the underclass (again, it's open season on zombies). I suspect in the 1950s zombies could have stood in for Communism.
What can we do with zombies that is different?
I have an idea. Postulate a near-future setting, for values of "near future" approximating 20-30 years hence. A cure for cellular senescence is found, and it's cheap. One injection, and your physical condition gradually reverts to where you were at age 20, over a period of years. It's not a miracle cure: it won't re-grow lost tissues, it doesn't cure cancer, it doesn't cure diabetes, it doesn't stop heart disease ... but if you can beat all of the above, you can in principle live indefinitely and in fairly good physical health. Moreover, it comes along at the same time as much better treatments for cancer and cardiovascular disease, expensive treatments to re-grow damaged organs or limbs, and the ability to clone up a new pancreas from stem cells.
However, there's a disease that some researchers are referring to as Type III diabetes (warning: PDF). It's a neurodegenerative condition caused by abnormalities in IGF-I and IGF-II signalling mechanisms in the brain, and may be the underlying cause of Alzheimer's Syndrome. And nobody's got a cure for it. (The "clone up a new brain from stem cells" approach is generally considered a bit of a non-starter.)
So we have zombies. They look like fit, healthy 20 year olds ... but their minds are gone. They don't eat flesh and brains, and they're not infectious. Mostly they just shit themselves and groan deliriously. They die if they're not looked after: they can starve to death sitting in front of a full plate. The process of turning from a human being into a zombie takes years, and the early stages are marked by emotional instability and sudden rages, which leads to major social problems: when a dementia patient is as physically fit as a 20-something and cuts loose, they can do quite a lot of damage damage. (Unlike a pre-anti-senescence treatment 90 year old.) Extrapolation suggests that most everybody who makes it past 110 years of age will suffer from dementia; projections show that in another 30 years, fully 20% of the human population will be zombies. Then, some time around 2100, the demographic bulge left by the first half of the 21st century will hit 80, and everything goes to hell in a handbasket: by 2150 we'll be up to our collective neck in zombies.
We keep zombies around for three reasons.
Firstly, there's the faint hope that there might be a cure somewhere around the corner. In which case, it might be possible to restore some of their old personality and memories.
Secondly, we can't trivially dispose of them: they're our parents and grandparents, people we love, people we owe. (This is the point of rupture, where this scenario totally departs from the classic zombie story: the point where we throw away the inhumane excuse for a chainsaw-assisted gore-fest, and start looking for a new metaphor. The zombies in this scenario aren't disposable "others", they're people we know and treasured and can't now dispose of -- a metaphor for memories and past ties and ended relationships, the intimately known rather than the unknown, and for being overwhelmed by the dead past.)
Thirdly, if we don't discover a cure, sooner or later we'll be joining them ...