Comment by WorldMaker

It can be preventative against spam for when old domains expire/get sold and/or old blog service passwords get hacked.

I think about doing that sort of proactive cleanup sometimes. There's nothing quite as disappointing as seeing an old friend's blog show a new post for the first time in years only for it to be some spammer that just hacked their old password or some expired domain squatter saw RSS logs and decided to sell advertising on it or a once major blog host was sold to a Russian oligarch who purged the user database so more Russians could have good usernames (LiveJournal, lol).