I came here to say that too. Scraping has a great many legitimate uses. Search engines, scientific research, trying to use publicly available data that doesn't have an API. I've had to scrape government websites quite frequently because they often make public information hard to read by other means.
That last one is an interesting one. I think one of the most effective way to deter a scraper might be to just provide an API!
Now if you were using the scraped data to republish (copyright infringement) or use it to gain a competitive advantage (re-pricing in eCommerce comes to mind) that is a different story.
... which rule would be obeyed only by legitimate, robots.txt-honoring crawlers. This reminds me of the anti-piracy messages shown (solely) to viewers of legally-purchased media. Similar "logic", similar (counter-productive) "effectiveness".
Re-pricing is the practice of scraping your competitors and pricing your product just a little lower so that in price comparison searches you always show up at top.
That last one is an interesting one. I think one of the most effective way to deter a scraper might be to just provide an API!
Now if you were using the scraped data to republish (copyright infringement) or use it to gain a competitive advantage (re-pricing in eCommerce comes to mind) that is a different story.