At first, I thought lingerers were kind of rare in zombie lore, but the more I look around the more I think they are basically the norm. Lingerers are zombies who retain something of their old selves. They remember something, either an ingrained skill or maybe even all their memory.
In the Romero movies many of the zombies slowly began regaining their human skills and memories over time. In Return of the Living Dead they were basically just angry versions of themselves.
Regardless of the film series, these are some of the most common zombies around.