I'm not sure why one has the ability to vote a question up OR down; perhaps downvoting was introduced before the closing of questions was. However, it seems to me that downvoting is redundant on the main sites. Surely the purpose of voting for a question should be to say "I like this question", or "this is a good question". If people don't like the question, why does it need to go into the negative numbers? What's wrong with it just staying on zero? The other purpose downvoting is used for is to 'suggest' that the question is inappropriate and should be closed. However, I don't think downvoting is needed for this. Why not just vote to close, or flag the question, in that case?
Note that I am drawing a distinction between downvoting on main sites and on meta sites (using downvoting for "I disagree" on meta sites makes sense, but this need isn't there on main sites). I am also drawing a distinction between questions and answers. Obviously downvoting answers is needed to be able to say 'this answer is bad/wrong'. This doesn't apply to questions.