When a thread is as busy as this one, though, there can be multiple conversations going on simultaneously, some repetition is necessary. It can get confusing that way, but it would also be confusing without it. Sometimes it's good to be reminded of the flow of conversation, otherwise, it can seem stilted and out of context.
Also, if you want to be able to have nested quotes (handy!), it can be tricky simply to quote one particular thing. Takes a lot of editing, etc.
SO-- it's a nice idea in theory, but not as practical as it sounds. Ideally, people would take the time to edit the quotes down a bit, but I find it simple enough just to not expect to read every last word anyway.