I wish I had the optimism to think the most voters vote for something rather than against it.

I don't think voter pep talks, however altruistic they might be, will do the trick. Given that tendency voters have to vote against something, I don't blame a candidate to be defensive. Clearly the Conservative strategy is to run the campaign with minimal mistakes. Hence the 4 question rule.
The problem is that gamely being open to questions is the same as foolishly exposing yourself to attack. Lets face it, the media hardly can claim innocence and objectivity in their motives and stories. This media has long been hostile to Mr. Harper. So he had to choose a side between the devil and the deep blue sea.