Consider this scenario: You've created a family. Let's call them Family 1. You play with them for a little bit. You create another family. Let's call them Family 2. You'd like members of Family 2 to interact with, perhaps romantically, Family 1. However, while you're busy playing with Family 2, Family 1 stays locked in time, never aging. This makes playing two families and trying to get them to grow up at the same time take FOREVER.
Enter "autoplay". You play Family 1 a bit to give them their bearings, then ask Sims 3 to put this family on "autoplay". While on "autoplay", Family 1 continues to age, interact with each other, all that good stuff, as if someone were sitting there actively playing with them. That way, once Family 2 member ages from teenagerhood to adulthood, his love from Family 1 is ready to do the same, and they can move in together!
Other exiciting things that would be cool about "autoplay": If families could continue to have babies, move characters in and out, and aquire and quit jobs while on "autoplay". Imagine how much fun it would be to do a Legacy Challenge where all of the non-heirs can move out, yet continue to age and live their lives, so that the original Legacy family could have lots of relatives! I think that would be awesome.
"Autoplay" would, of course, be an optional feature, as there are drawbacks to it. Such as, your Sims running off and totally messing up your carefully laid plans while on "autoplay". But I think it would be a totally badass feature that would make gameplay a lot more interesting.
It's too bad I'm not programming for Sims 3.