You've seen people do it. If you run a forum and try to keep a tight ship, you hate it. People that quote the entire previous post in their own. It makes no sense.
And it's not the people who “grew up” on Usenet either. I ask you, when you verbally reply to someone, do you repeat every last thing that they said to you before you get on to your point? Are people lazy, stupid, or just don't get it?
I wouldn't say it's something that makes me “angry” (I can think of more important things to be angry about), it's just annoying.