Let's face it, just about anyone who is an undecided voter at this point is... well, not especially bright. Some are genuinely stupid, and some may be smart people who just don't think paying attention to current events is a valuable use of their time. Obviously from the perspective of predicting election outcomes it's useful to know what these people are thinking. However, it's not clear why handing a debate over to 80 of them and letting them write the questions (chosen by Brokaw, of course) is supposed to enlighten the rest of us.
That's all true, and let me echo that. Pandering to undecided voters makes the assumption that decided voters are irrelevant. And we're not. Here are some choices a decided voter can make, depending on how a candidate behaves.
Change one's mind from one candidate to another
Change one's mind from voting for a candidate to not voting
Get enthused about a candidate such that you tell your friends and family to vote for him/her
Get discouraged about a candidate such that you stop telling your friends and family to vote for him/her
Volunteer for a candidate
Give money to a candidate
Decide to vote for every candidate on that party's ticket
Decide to split your vote between the Presidential candidate of one party and lower ticket candidates of the other party
Hold a fundraiser for a candidate
Write to a candidate's campaign to express interest in an issue or answer
Pandering to undecided voters (who incidentally may not be undecided - we humans are notoriously bad at understanding how we make decisions) leaves out the most interested and passionate people in our political system, those who think about civic engagement in multiple ways aside from just voting. It assumes there is no way that anyone could change their vote. And it assumes that the Presidential election is the only way that voters engage in the political system.
And that's just a argument for why decided voters are relevant and should be part of an electoral strategy. There are also plenty of reasons to think that undecided voters ask ridiculous questions that aren't relevant for the rest of us, but that's a different question.