An esteemed professor of government I know wrote a paper several years ago about communications at the center. It was about the Canadian context, but many of the principles are the same. The takehome message was that the proliferation of electronic communications has upped the ante when it comes to the difficulty of presenting a coherent front to the political, media, and public world. The sheer millions and millions of occasions in which the world attempts to solicit a response from the government, and the government attempts to communicate, make coherence of off-the-cuff statements very difficult to maintain. In the midst of the twitterverse and blogosphere, and all the rest of it, communications strategists have ascended in importance, and rigidity of message becomes a default strategy, lest someone try and improvise and mess up.
But I hear what you're saying. All too often the people we hear from are those who have practiced the approved media lines most, and can say them in their sleep. There are times when one longs for a little authenticity.
At the same time, elected officials have to be constantly vigilant for sound bites that may well come back to bite. The slicing, dicing, and julienning of verbal interaction with officials means that you will NEVER hear a video segment from a poltical interview that might contain the phrase "But getting back to your question...". Perhaps the complaint is not with the government but with the folks in media who filter the content and insist on content that will stand alone within a 10-second or one-printed-paragraph space. Maybe we get rigid media-line adherence because that's what works in the world of e-media? Maybe the nuance and articulation you and I both crave are simply less compatible with the twitterverse than repetition and clinging to message.