I've never played a premade Sim, but some of them become important parts of my Simworlds. This happens when one of the premades connects, organically, with my "main character."
My current favorite Sim has several fun connections with premades. He met Brent Hecking at the gym, and now he's Good Friends with Brent and Brant. He hangs out with them a couple times each week. He clicked almost immediately with Summer Holiday, and since the Sim I created to be his love interest doesn't seem to like him much, his crush on Summer could easily turn into something more. He's also become friends with Dominic Fyres, Bjorn Bjergsen, and Simon Scott.
Another Sim I've enjoyed has a deepening connection with Sofia Bjergsen. She is still a Teen in my save, and he's a Young Adult, so they have to wait for romance, but their chemistry is both sweet and electric. There's definitely a future for those two.
Nancy Landgraab seems to find every one of my single male Sims. One or two of them have had flings with her. At one point, I thought she was out to hook up with my current "main character." Nancy's intentions have so far been platonic, however, and they simply enjoy talking and working out together.