I can think of plenty of users with >500 merit who would quite happily abuse this and brigade another user's posts to have them all hidden, especially in the case of trust disputes. Also, such a feature would undoubtedly be misunderstood and used for posts the user dislikes or disagrees with, not just posts which are redundant.
Do you really think users would collude like that? I mean, a threshold (of distinct and qualified members) has to be reached before the post is considered "redundant". The merit requirement (and/or DT status, mentioned at the end) is there to mitigate abuse (with farmed accounts, etc.)
Other systems that the forum already has, such as trust, sometimes suffer abuse, let alone this one.
Then, the fact that the post is hidden in the topic, it will not disappear from the list of posts made by the user, which is probably the list that campaign managers analyze.