First of all, I cannot =prove= that these site have effected me negatively, at least not with figures ("I lost XXX due to scrapers").
But after having seen countless scrapers as a web user and also as an alert webmaster, I came to the following conclusion:
1) Scrapers do =not= provide any useful service (which is the difference to real search engines). They are so utterly ugly/bad/useless that any user who sees it either hits the back button or is being tricked into clicking one of the numerous ads. This reveals the real intention of the scrapermasters: Earn money (which in principle is fine, BTW).
2) They do it off stolen content, i.e. without adding any content or service on their own. A software creates thousands of useless pages containing just gibberish, fine-tuned to show up in SERPs, finetuned to trick users into clicking the ads.
3) So we know they do it for the money, and they use stolen content. But, you see, advertisers have limited budgets. Every single click for a scraper pulls money out of the advertisers pocket. Money that otherwise would be spent for ads on sites belonging to the real content owners. So I assume that the community of honest AS publishers is being hurt =collectively= by the community of AS scrapermasters with their zillions of useless pages. And that's why every honest webmaster gets emotional when it comes to scrapers...
We can not prove it, but we know that they hurt us.