I agree. Answers with not much more than blocks of text copy/pasted from other sites can take away from the quality of the site. The SE model is designed to bring people in to see our work. Using outside sources is integral to that, but not at the cost of our originality. We want people to find our answers when they do an online search, not copies of things we found online with very little else.
The How to reference material written by others page in the Help Center already addresses this issue.
Do not copy the complete text of external sources; instead, use their words and ideas to support your own. And always give proper credit to the author and site where you found the text, including a direct link to it. (Emphasis in the original.)
Long quotes often include extraneous information. Even if some material is correct, much of it gets buried. They're a different color, and usually a smaller or different font, so it's harder for me to read. When the quotes are broken up by some of our own wording, it's easier to go back and forth. It also keeps the quotes matching the context in the flow of the answer.
How do I write a good answer is another helpful resource from our own Help Center.
Links to external resources are encouraged, but please add context around the link so your fellow users will have some idea what it is and why it’s there.
From what I've seen, primarily cut/paste answers are usually the first ones and get the most votes and acceptances. That can deter people from writing something that's more original but takes more time.
As Rory Alsop said, there are exceptions, and I think we should, and can, find a balance. There's a place for those answers, but I agree that in general we should keep them to a minimum.