Frankly, I think this rule is an unequivocally terrible idea. We already have standards for harassment that could easily be met by unexplained strings of low ratings. For other ratings, we have no right to say that some people's honest opinions are more valuable than others.
I've said this before in the staff forums to no effect, so I'll share it with a larger audience now: we should do what YouTube and other sites figured out a long time ago and switch to a like/dislike system for all our database content. As Saffith said, a five-star scale means different things to different people, but in general, people are self-consistent when it comes to what they like or don't like. The added benefit is that people with extremely low or extremely high standards don't appreciably throw off the average by giving reviews that top-out or bottom-out the scale—because everybody has the same power.
A like/dislike system removes the need for this rule and all its dangerous implications.