It's useful when the quotes are from a several posts back or a different thread or long threads.
Yes, there can be abuse where users quote a whole post which is -1 or -2 posts up, and add just one line as response. This doesn't happen as much on LQ as on other sites but it can & does. So on LQ, it would be useful.
Another useful option: multi-quote. You select which posts you want to quote and then click 'quote'. Result is a series of quotes which aren't nested.