I think there's a difference between "contrarian about X" and "contrarian". The former has (hopefully) looked at the evidence around X and come to a position on X that differs from the mainstream. The latter values being different over being right.
I think the first sort can be valuable, and shouldn't be driven away.
Wei Dai's first sentence only talks about the second sort, and I wouldn't call someone who has come to a position on X that differs from the mainstream a "contrarian about X". If they call themselves that, then instead of simply being able to present their arguments, they have tied their identity to being in opposition, and the whole downward spiral I described comes into play.
I'm worried that LW doesn't have enough good contrarians and skeptics, people who disagree with us or like to find fault in every idea they see, but do so in a way that is often right and can change our minds when they are. I fear that when contrarians/skeptics join us but aren't "good enough", we tend to drive them away instead of improving them.
For example, I know a couple of people who occasionally had interesting ideas that were contrary to the local LW consensus, but were (or appeared to be) too confident in their ideas, both good and bad. Both people ended up being repeatedly downvoted and left our community a few months after they arrived. This must have happened more often than I have noticed (partly evidenced by the large number of comments/posts now marked as written by [deleted], sometimes with whole threads written entirely by deleted accounts). I feel that this is a waste that we should try to prevent (or at least think about how we might). So here are some ideas: