Voice search will not be actually useful until it can respond naturally to naturally voiced questions. The whole point of voice UI is to make things more natural for human interaction. If it's just a matter of how well computers can interpret speech, well, that's a fun parlor game, but it's not what is implicitly promised by a voice UI, and it's definitely not what's explicitly promised by the marketing for these services. And ultimately until these things can interact naturally, they are doomed to being a novelty.