Spam. We're all seen it. We've all seen deleted posts from it.
But sometimes even after the post/user is deleted, the spam remains, like in this screenshot below:
When you QUOTE the original spam post (especially if it contains links), the bad content does not get deleted when moderators delete the offending post. The links to advertising/shady/inappropriate websites remain for all users (and web search bots) to see.
Thus, I would encourage all users
DO NOT reply with quoted text to spam; just post 'Reported' by itself or delete all the content from inside the QUOTE vBulletin tags. And for moderators, I would encourage them to also delete any posts like the screenshot above to fully wipe out spam.