But, maybe they should also based the rankings on something other than your posts (e.g. helpfulness of posts, posting rate, time spent online, etc.)
I agree that in theory this would be better. But how would you implement it? None of these criteria are objective. Some are easy to influence.
For instance about the subjectivity of "helpfulness of posts" : Most experienced posters will create posts to make the person who asked a question to reflect about the subject. In other words: "They know the answer but they will only give a hint to have a learning experience." (Give them fish they have food today. Learn them to fish they have food forever.)
This is perceived very helpful by all; except -very often- by the person who needs the answer.
About "posting rate": The moderators do a tremendous effort keeping this forum spam free (and they do it for free). In this situation you do not want people to start posting just to upgrade their "tag".