I love abduction babies so much, I used MCCC to increase the odds and let female sims get pregnant from abductions. I'll also give them extra alien markings during the duration of the pregnancy as a bonus side effect. At this point, bonus alien babies are just an occupational hazard for my scientist and astronaut sims. I think the last time I played an astronaut, 3 of her 7 kids were from abductions.
It's also super cute when a couple is pregnant at the same time; him with the alien baby and her with his baby. It's like bonus twins. Weirdly enough, I never actually play the alien kids once they grow up; they usually just chill as background characters for the human siblings.