I can definitely see why you'd think this could go the way of Facetime, but its completely different. Why? Because while Facetime seemed to solve a problem we all wanted solved, in reality its simply not practical to video call someone with a mobile phone a lot of the time. If I'm on the go, walking a city or driving in the suburbs, Facetime is a terrible idea.
Assistant is exactly the opposite. It's almost essential in those situations and extremely practical. I can pretty much see Oprah endorsing this feature alone in her anti-texting-while-driving campaign now.
And YOU get an iPhone Assistant, and YOU get an iPhone Assistant...