I agree, whereas zombies add humor and gross factor, making simming feel a little less painfully pastel and babyish, frankly soms3 zombies were an intrusive pain in the neck that couldn't be turned off and I still find them 99% annoying rather than worth it. They should be like other occults: something you can manage so that if you don't like them, you don't have to deal with them.
I'm only in a zombie mood once in a whe and the rest of the time they are like having your neighborhood walk ruined by irresponsible dog owners who leave the neighborhood strewn with dog waffles to dodge. They, and people who double park (truck owners most often) have no idea how thin is the protection they enjoy from retribution, based on other's decency. Which only stretches so far before it snaps.
But zombies are a literal plague in sims3, and if they became an option in later games, they need to be an OPTION. They are beyond tiresome but only because they can't be turned off or managed and contained.
No other occult gets to be that intrusive and annoying, and this keeps the peace between simmers who want mayhem, and simmers who want Perfect People in Happy Valley. I like me some mayhem, and those zombies wear out their welcome fast.