First, a confession: I actually watched a few minutes of Tuesday's Ohio debate between Senators Clinton and Obama. I don't usually watch the debates, which doesn't make me especially virtuous. I just find them to be a combination of boring and sickening. But I managed to watch for about fifteen minutes on Tuesday. Here's what I noticed: Tim Russert (who was asking most of the questions while I was watching) is very good at trying to pin the candidates down. This is not entirely a good thing.
If a candidate, or for that matter, an elected official who is not in the midst of an election campaign, says one thing to one audience ("I love NAFTA," for example) and the opposite to another audience ("I hate NAFTA"), the press does its job by pointing out the inconsistency. This enables voters to realize that the candidate is insincere---itself valuable information---and also to look for other sources of information about what the candidate will actually do. Good interviewers ask follow-up questions (as Russert does) to pin down politicians who are otherwise trying to appear all things to all people (which is what most politicians want to do much of the time).
But there are some times when politicians legitimately don't want to take a public position. Take the NAFTA case. Russert got both Clinton and Obama to say, under pressure, that they would threaten to withdraw from NAFTA if Canada and Mexico don't renegotiate its terms. Neither Clinton nor Obama exactly said that she or he would carry out the threat of withdrawal, because each said that a credible threat would induce Mexico and Canada to renegotiate.
I took from their answers that both Clinton and Obama have the following preferences: 1) Renegotiate NAFTA; 2) Keep NAFTA as it is; 3) Withdraw from NAFTA. Forget about whether this is the priority of preferences that Clinton and Obama actually hold, and also forget about whether this is the right preference priority. Assuming I'm correct in my assessment of what they actually think, and that they're right to think so, is it a good idea for a journalist to dig dig dig until we get the answer from the politician? Of course not. Revealing to Canada and Mexico that the worst outcome for the U.S. would be withdrawing from NAFTA renders the threat to withdraw not credible, and thus eliminates pressure on them to renegotiate. Here, the politicians' preference for caginess is not an effort to deceive voters but to maintain a strong bargaining position.
Transparency from our elected leaders is generally a good thing, and evasiveness is often duplicitous, but not always.