They also help to respond to different parts of the last post separately.
If you refer to a specific part, it's ok to quote this particular fragment or the most important keyword you focus on in your reply; but it doesn't make sense to quote the whole post - you can see this last post above the post form, so you don't need to duplicate it. Look at it from the reader's perspective - he/she doesn't have to read the same message twice and by reading the previous message it's easy to get the context of the next message.
We made it easier to see the last post - now when you click on the reply / quote button, the 'Similar discussion' links should disappear so that posters can see the last message in full.