Yes. They should be deleted.
There are many reasons why copy paste answers are not useful. Some of them are:
Lack of effort.
These answers do not show any effort. If the answers are just a copy paste from an external source, what is the need of our answer? Why should a reader read your answer? Providing a link to the original would be enough. If you say finding a detailed article is also effort, then people good at google foo can write good answers in <put site name>. But we want users who know that topic and can write questions and answers in that topic in detail.
That doesn't help the reader
Your answer should clarify doubts of the reader. One of the reasons for deletion of low quality answers is something like "Write answers which do not require further clarification from the OP". But here on SE, the deal is not only between the OP and you, the community should also understand the answers. More importantly, the readers of the content should understand the answer. A copy paste answer would not help the reader as there is nothing except a giant block quote.
We want your answer
People ask questions here because they want your answer. If you are just copy pasting from external source (even though it is properly attributed), it is someone else's answer. You are reproducing here.
Always explain the answer even when you are quoting.
Pure copy paste answers would just clog up search results. Shog9, a community manager, wrote the following about posting original content:
And worse yet, an awful lot of them are answered that way: by quoting external sources. A site that just regurgitates things that others have written isn't much good; answers like that just clog up search results without adding anything. It's important to cite your sources, but just as important to demonstrate knowledge of both the question being answered and the topic itself - this is where we can actually make the Internet a better place, demonstrating how to use this knowledge to answer specific questions.
Even when you are citing a source, try to paraphrase it in your own words by using their ideas. Use some portion of quotes rather than everything.
Explain the answers in simple terms
One of the mottoes of Stack Exchange network sites is to build a library of detailed questions and answers. We should try to write answers like experts. An expert always tries to explain things in a simple manner. Some post questions because they don't understand complex terms used in the topic. If your answer uses the same, then that is not a good answer. Your own explanation is needed in the answer. At least some introduction or explanation of terms.
A copy paste answer, even if it's properly attributed, doesn't become a good answer.
If you say only plagiarism is discouraged and complete copy paste is ok, then there is something about referencing material written by others too. The help center article about referencing says
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.
These are some of the reasons copy paste answers are discouraged and deleted. This topic has been discussed in many communities. Some have included it in their faq too.
If an answer is only a copy paste from external source, providing a link to it in the comment section is enough. I see that the moderator did the same. Don't worry. If you paraphrase the answer in your own words, you can flag the answer for undeletion.