It's a recursive issue, and this is something that Reddit could potentially improve. Reddit preserves a deleted comment's space in the comments section as [deleted]. Users who see a graveyard of deleted comments will comment to wonder why, especially if they are unfamiliar with the subreddit rules. This may require the mods to delete more off-topic comments, which worsens the issue, etc. So the established "best practice" is to delete but leave a moderator comment with an explanation so there's a paper trail.