Let's look at it this way...
When I want to find a campaign to play, I load up the campaign list on the Wiki. I read the description, the requirements, and any veteran comments. I get far more out of the veteran comments than any numeric rating. Furthermore, if something is rated poorly, I want to understand why its rated poorly.
Thus, this is my conclusion - keep the campaign listing the way it is. It works just fine, and if you are wanting to play something, you are more than capable of using your own judgement in determining if you'll like it.
Otherwise, if you must include a numeric rating with a campaign, any voter must include a comment with the said vote. This might be comparable to ratings you find on iTunes or other such download sites. In that manner, you'll help prevent any empty imput for or against a campaign; empty imput does no one much good, after all.