[supreme_loquat]

The biggest issue with a proposal like this is that a lack of any preference given whatsoever should not count positively towards a candidate. If someone labels candidate A as +3, candidate B as -3, and doesn't label candidate C, candidate C should not be given more preference than candidate B. People will either not understand the ramifications (they might not know someone and think that "no preference" is truly no preference when in reality it's positive preference), will forget, be lazy and not fill in some candidates, etc. I don't think that's a desirable trait since it's essentially ascribing a vote which the voter did not want. Of course if you want to make -3 (on a -3 to +3 scale) the default value assigned to "no preference" votes, perhaps that would be okay.