I feel that the current "delete on sight" policy is still the best course of action. It's important to remember that there's nothing stopping the poster from properly attributing a copy-paste and re-posting after it's been deleted, or if it was targetted in error from arguing against the charges in meta to get the post undeleted.
The problem with any sort of grace period is that it effectively just means the problematic post is visible longer on the site. In my experience, and completely aside from the general problem of bad content lowering the apparent value of the site, the longer an unattributed post is on the site the more likely it is to:
- Be edited in good faith for apparent [spelling/grammar/content] issues by users who don't realize it's quoted text; this makes it not only more difficult to find the original source, but would all need to be rolled back when the post is properly attributed and quoted (or scrapped entirely when the quote is summarized instead of quoted) which just wastes everyone's time.
- Attract comments seeking clarification or providing constructive criticism on the text, under the impression that the poster who posted it had anything to do with writing it. This, again, wastes everyone's time since the person they're actually trying to criticise and/or seek clarification from is not the person who posted the article, and likely has nothing to with the site at all.
- Attract votes based on the content of the article, again under the mistaken impression that the poster had anything to do with it. In my experience, I've seen users earn hundreds of reputation points from plagiarised articles before the plagiarism was caught; this not only gives them the mistaken impression that this sort of activity is in fact encouraged, but risks upsetting the whole "reputation = trust" model (should we as a site really "trust" someone with site moderation abilities we can't even trust whether or not they even wrote the answer?)
For low quality posts in general, the above issues of editing/commenting/voting would actually reflect, and possibly improve, the content of the author's post, in which case they'd all be working as intended. But when there's a failure to quote and attribute, when the author's actual content (if any) is indistinguishable from third-party text, I see far more harm than good from allowing any sort of grace period.