While there may be some exceptional cases, I am against a general practice of deleting bad answers.
First, if an answer is heavily downvoted, then a viewer clearly see it is not credible (and it actually that the community is healthy). So I don't see a particular advantage of just erasing it (unless its author opts for it).
Second, for me the whole idea of SE is voting by the while community, not only the moderators. One can add further Why shouldn't we delete any wrong answer, no matter its score? But such lack of trust in the community does not seem to be a proper choice.
Of course, if an answer is very off-topic (in the relation to the question or to the whole site), or generating heavy rants, then deleting it may be a good option.
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