The premades and townies are almost always secondary characters. I sometimes add in more that I don't intend to play .. just put them in houses for extras.. they can be sims my friends will make friends with, have in their clubs... background mostly. If my sims get to know them well enough and it feels right they might become a main character .. usually by moving them in.
They also serve as the victims when I engage in deviant play. I've played with most of the premades in many ways .. poor things. I do have favorites but that's mostly the ones my sims come in contact with more frequently because of where I tend to make them live.