The biggest concern with copy-paste answers — aside from potential copyright infringement which wouldn't really be an issue here since all SE posts are CC BY-SA 3.0 — is that they are often if not always posted by users who have pretty much zero critical understanding of what they posted and, as such, zero practical ability to respond to criticism, provide clarification, or otherwise improve the post when called to do so.
For the most part, answers are expected to be accurate, complete and self-contained; when users are leaving constructive criticism or seeking clarification in comments, and these are not being addressed, then the answer is likely not meeting these criteria.
And of course, if the user is deleted then there's nobody around to address these concerns; even if users are willing and/or able to correct the answer based on their own knowledge (rather than the author's), making substantial changes to another user's post is just bad form.
Most of this wouldn't be an issue if the answer was community wiki, since CW posts are intended to be worked on by the community as a whole rather than exclusive to the original poster. However, in my book changing an existing answer to CW when the original author didn't intend to do so counts as exactly the same sort of "substantial change" that should be avoided.
And as for the possibility of taking the existing (deleted) answer and posting it as a new CW post, that may have some merit. However, if the poster (or the community as a whole) has enough knowledge of the subject to actually respond to criticism, provide clarification, or otherwise improve the post when called to do so, why can't they just write a new post in their own words and post that as CW instead?
As for how to undelete an answer, any user with sufficient reputation (4000 on beta sites) can cast an undelete vote exactly the same as a delete vote. This, however, cannot be done if the post was deleted by a moderator, including the Community ♦ "user" which is responsible for deleting posts from deleted users.
The only way to undelete a post deleted by a moderator is by convincing a moderator to undelete the post.
So now the big question is "How can I convince a moderator that the answer has value and should be undeleted"? To which I will rebut with another question: Why was it deleted in the first place?
Even when an account is deleted, the Community user would only delete their answers if they already had a negative score: This suggests that at least not everybody thought it had value as an answer.
Moderators, for the most part, can't be assumed to have domain expertise: We are poorly equipped to judge whether an answer is correct or incorrect. In many cases, the only real criteria we can use to judge a good answer from a not-so-good answer is the same as for every other user on the site: Voting.
As such, it's a rather uphill battle to convince us that a post which was negatively scored, written by a user who no longer cares about the site (or, by extension, the answer itself), is worth undeleting at all. Chances are, it's not.