There are different scenarios when we quote from an established source.
(1) To share something we find useful. In this case, if the reader does not agree with the idea, he is disagreeing with the source, not the one who shares what he finds. This will be valuable as the writer will view the materials he wants to share from a different angle and thus broadens his vision. Of course he can also defend the idea to further explain it to the reader. A fruitful debate will spark off.
(2) To support his idea. In this case, he must have his own idea and what he quotes is just to show that his idea is supported by (not copied from) an authoritative source. Such quotes are necessary as a new idea has to be hatched from an egg and not burst from a rock.
Having said this, I want to comment on some people's attitude. When you present an idea, they would demand that you quote the source from an authority. When you say it is your idea , they are not satisfied. They will not accept your idea even if it is logical. They will rather accept a false idea when it is directly quoted from an ancient book. They will not be satisfied even if you quote from an authoritative book to support your idea. This is a very wrong attitude.