On all mailing lists I'm on that allow non-subscribers to post (a few high-profile FOSS projects) the benefits far outweigh drawbacks. A few individuals period

On all mailing lists I'm on that allow non-subscribers to post (a few high-profile FOSS projects) the benefits far outweigh drawbacks. A few individuals periodically complain about reply-to policy and duplicate emails (which, if you're reasonable and take a few minutes to set things up, are non-issues)... Spam is, mostly, minimal.