Voice interaction works great in situations where you either don't want to, shouldn't, or can't interact via touch. These situations also tend to be the ones where you don't want the distraction of any computer device (eg: driving, sports, etc.).
My biggest bias here is that I don't drive. Talking loudly to your phone feels way more intrusive in a packed train than in the intimacy of your car, even when you're stuck in traffic.
