Was Julie the main, long-term reason why Chrissy was added?
Being that she was introduced as Chrissy's nanny and all (besides the whole rationale of, "We need to add a new kid to freshen things up!") but the whole endgame (whatever it was originally intended to be) was screwed up when Kirk Cameron got Julie McCullough fired for posing for Playboy.
I mean, when I think about it now (having seeing some of the episodes on the Up channel), Chrissy didn't really effect or harm the storylines too much at first. It wasn't until the producers had the infinite wisdom to "age-up" Chrissy by hiring the irritating Ashley Johnson and her cloying cuteness that it really became problematic.