1. It's usually unnecessary.
2. It's bad form to quote a huge original and add a couple of words at the bottom.
For me this is a non-issue, as you can use the quote button and delete the text that isn't relevant to your reply.
I also disagree it is usually necessary
doesn't include User name
In short I may only agree/disagree with the part that I quoted(with the user name included) and being able to do so conveniently(due to the button) is helpful.
I will however concede that it'd be great if people removed non-relevant text when they use the quote button.