This answer has a score of -6 and a message that Unsourced material may be challenged and removed. This post is from 2011. It hasn't been removed. Personally, I like that it's not removed: seeing good and bad posts is a good way to learn about the site, and sometimes bad posts can still be interesting to read. I don't have the reputation to see deleted posts. Is the message may be (...) removed actually followed, or is this an empty threat?
To add to Fabian's answer. Unsourced answers are not bad answers. They are not answers at all by virtue of the site's scope, which is to review the evidence.
We do leave bad answers in: answers which have poor references.
It may sound like nitpicking, but there is an extremely important point. We don't delete posts based on right/wrong or good/bad subjective judgement. We remove posts based on an objectively verifiable criterion (lack of references). Hopefully our impact will then be better signal to noise and not a bias.