I can think of several occasions when I would have used the reorder option in discussions on my forum. I've collected together different posts I wanted to respond to and I've changed the order of the quotes and my responses because some points were more relevant and important than others, so they were moved higher up.
As for the remove option, it seems daft not to have it. As multi-quote is so easy to use, there will bound to be the odd time something is added to multi-quote by accident. The other example I can think of, and I've done this with the 'old-fashioned' copy-posts-to-notepad approach, is I've wanted to respond to someone's post so copied it, then later found that someone has already responded to the post with what I wanted to say, so there was no longer a need for me to do it. I then deleted the copy of the post from my list.