Imagine the workplace. Right now, if I hear anything from the surrounding cubes, it's a tap-tap-tap of keys that fades into the background.
Should someone be on a phone call, or heaven forbid, speakerphone, it's an active distraction.
Now imagine everyone around you talking to their computers. You'd never get anything done, especially if someone is dictating a requirements document. The horror...
Speech will never supplant the keyboard for the vast majority of current computer usage.
This isn't saying that some new paradigm or use won't come up. There might be something that uses speech to supplement something that you're using your hands and/or feet for. Changing the music or using your turn signal while driving, for example. In effect, allowing you to use your voice as a third hand.