Voice search on the phone is natural. The phone is a device that historically is good at one thing, voice. Even though a modern phone has a decent keyboard input, t's still clumsy when on the go. When I'm out for a run or a ride and I want quick directions, or to dictate a note, send a text message, or check the train schedule, the voice interaction is vastly superior to wrestling my phone out of it's armband and typing something. The voice interaction isn't amazing, but it works about 80% of the time on the first try and that's good enough for me. I don't have to stop my workout and fumble around.
On the other hand, when I'm sitting at my desk I can, with two key strokes switch to my web browser and launch a new search tab. I can type about as fast as I can speak and my accuracy is probably around 95%; google makes up for the remaining 4% in spelling errors (searching for instead of ). I get better accuracy and less fumbling around if the room is loud. Also, in our quiet open-plan office I look like a total D-Bag talking to my monitor. That's a big plus. too.
The full-size keyboard isn't the end-all of interfaces, but for a desktop it's waaaay better than voice search. If voice search ever gets to the point where I can throw out a complex, natural language queries into the air a la ST-TNG, I'll switch. "Computer: Post a witty comment to SlashDot about voice interfaces and how bad they sucked in the naughties and teens"