I mentioned something like this before but had to be content with the current system. The best use of multiquote I have seen is as VC says.
You scroll through a thread, reading posts as you normally do. As you're reading, if you see something you wantto reply to, hit 'quote' and carry on reading. After hitting 'quote' for the last text you wish to quote, hit 'add reply' and all of the selected posts are quoted. Very useful in multipaged threads and a lot easier than having to copy and paste individual quote replies.