Here are two examples for why the warning is useful.
- If the original text contained a word or use of a word that IS prohibited, and Moderators come in to take action, then that post is likely going to be deleted.
If that prohibited text is quoted in entirety later in the thread, then that post with quote will also necessarily be deleted. Whatever content the "quoter" offered is lost as collateral damage of the clean up.
Given this possibility, a member should in fact receive an intelligent warning of this.
When I am the one making the deletion, I try to write a note to the member who's collateral post is deleted, so they understand "no harm no foul" but if it's a big messy one, sometimes this may not get done.
In that case, the member having had the warning, would hopefully be understanding of the situation.
- If the nasty bits being quoted are part of an online debate that has turned into a flame fest that violates the forums rules, a member that is engaging in that hostility is receiving warnings as they proceed, reminding them that they may be straying into moderator action.
I see no problem with an automated reminder that continuing could results in consequences.