Currently, automatic comments are always added when you vote to close as a duplicate.
However, it would be nice if, for example, closing as off-topic asking a question or a review on not yet written code, would add an automated comment on the question informing the OP. I notice that regulars currently do it themselves and pretty much always copy paste the same content, so why not automate it? We're IT'ers after all.
One problem I see with this is that it'll clutter the comments section as the same comment will essentially be repeated up to five times. As @Simon has mentioned, an app can be used to post customizable canned comments when voting to close. It really just takes one comment to inform the OP.
This is also different from closing as a duplicate because votes for a certain duplicate will automatically upvote the associated comment. This is only different when users vote to close as different duplicates, but that doesn't usually seem to be an issue.
If I'm understanding this correctly, I suppose you're asking if closing as off-topic can be treated the same as closing as a duplicate.
