While I can think of certain things that you might want to be able to do hands-free, these are mostly activities that don't have anything to do with the car itself. Hands-free radio controls, hands-free mobile calling etc. And I'd prefer that these features not be integrated into the car in a way that I can't choose to buy a car with, say, a different radio that doesn't have those features.
In other words, I can't think of a reason that a car should necessarily come pre-installed with any sort of microphone what-so-ever.
But the idea that there is not only a microphone, but that it is sending what it picks up to some remote server, is a notion that I am deeply uncomfortable with.
Most people view the inside of their car as being somewhat akin to the inside of their home. While people might expect less privacy in a car than in their homes, there is still an expectation of privacy to a degree. And that privacy expectation is typically more auditory than visual (of course you can get tinted windows but people expect you might be able to see inside of a car, but not necessary hear what two people are saying to one another).