The concept of quotes have existed for a great many hundred years.

During this time, it has been found to be a very well working method represent a dialog, keeping the focus on the intended subject.

Even when only a sentence or two is quoted, it would still be obvious which part of the original text the poster considered important. And it would explicitly indicate the relation between the two texts.

As a further bonus, a quote allows a reader to grasp enough context to remember older texts read even after quite significant time.

But the really interesting things with a quote is that if Google comes and indexes a thread here in tree-view form, that quote greatly improves the chance that Google gets all the important keywords all together. That would be good, since it gets visitors to this site. And to relevant posts.

A tree view where only the text from one post at a time is included, is extremely search-engine hostile. The post can contain an excellent answer to a problem. But be void of the magic keywords that represented the original question.

In the end, quotes would normally save bandwidth by having the reader see enough information in a single take to be able to continue without any backtracking refreshers. The main exception is of course the abuse sometimes in existence where a reader always takes full quotes just because it's "faster" than extracting the important sentences.