People feel bad giving a candidate -1, while others don't feel bad. The biggest help is a +1 for you and -1 for others. So there is a bit of an issue there based on how nice the person voting is, which can skew things. One person said they did not feel comfortable giving someone a -1 even though they support another candidate fully.

It's an issue because when compared to those who vote +1/-1, those who vote +1/0 are having a weaker voice.

Another thing is that candidates may get sad if they get a negative average score. (This is the same reasoning why some voters do not want to give a -1.) Psychologically, I think a negative score is much harsher than a 0.