A UI is a least common denominator that everyone can learn quickly. But it's nowhere near native quality. You can't get much more than simple one liners in trade talk. Good enough for negotiating a chest of cloves, but you'd be hard pressed to explain the difference between marxist-leninist and labour socialist perspectives on international commerce in a trade tongue.
When people hear "voice interface" what they think is a machine that can understand human so we can stop speaking goddamned pidgin all day. That is essentially a turing test, or some subspecies thereof.
I'm somewhat skeptical about this kind of thing. I'm confident that mechanical issues like translating sounds to sentences can be solved (the voice part). The translation of human to machine code… That's a different challenge.
It's an interesting case though, to have out in the wild. Siri and whatnot. An application (as opposed to an average starship computer) has a limited raison d'être, therefor a much tighter search space.
Meanwhile, we have been making consistent progress on turning our clunky pidgins into nice creoles, more elegant and powerful. Machines can understand higher level programming languages that people can get more fluent in. Average humans are generally becoming more computer literate (or fluent in UI, to keep my analogy straight). UIs are getting more intuitive, which basically means closer to human language. The common ground is expanding from just common ground and a wider language is being developed that accommodates us both better.
The question to answer is what part of that common ground is voice is open, I guess. But, I would wager that the common ground we find will not be human sounding anytime soon. It will be a UI metaphor, like the desktop. We will need to learn it.