We run a high volume site, with up to 18 posts per day by our writing team, and we close comments after several weeks, when posts are 20 or 30 pages back. Our old posts weren't generating any legitimate comments at all, only spam and spammy trackbacks. Our traffic is too high to use Akismet without paying through the nose for it, and managing an active community is time consuming enough without going through all the trouble to delete the spam comments manually.

If we could find a spam solution that worked without a bunch of false positives and cost lest than Akismet each month, we'd be all over it and leaving comments open, but it was a huge drain on all of us to deal with constant spam.

I haven't received a single complaint since we started closing comments on old posts, so it works for us.