What do you think about such answers?
Personally, I have a strong dislike for answers which quote other sources without offering even the merest explanation from the answerer. The value in asking SO questions comes from harnessing the combined experience, wisdom and eloquence of the community, and this is sometimes lost in the dry text of quoted sources.
I'd argue that there's less value in solely parroting (occasionally impenetrable) sources, particularly when those reading the question and the answers offered up (now and via future searches) will be of wildly different levels of experience and understanding.
For me, a quote from a source gives greatest value when it's used in support of a plain-English answer or explanation, rather than being posted as an answer in itself. This answer, for example, quotes the C++ standard to great effect, whilst offering adequate explanation from the answerer.
Should I flag this as not an answer?
Should I downvote it?
I'd stop short of flagging or downvoting, unless the answer fits the criteria as being incorrect, not-an-answer, etc. I guess my view is that a 50-line quote with no explanation which is correct is still a correct answer, even if it's being communicated in a poor fashion.