Threading vs linear posting is the issue here. We have threads which provides a single place for each discussion - it may wander, but it already separate from the other noise in the forum. We have linear posting, which makes it easy to figure out what's unread in any given thread.
In a strictly linear situation, such as newsgroup or email list, nested quoting makes a lot of sense (though email and newsreaders which attempt the thread such lists help here).
In a strictly thread situation (reddit, I'm looking at you) you have a really difficult time figuring out which comments are new without also rereading comments you read minutes or hours ago (though, not being a redditor, they probably have tools to deal with this more easily).
While I used to be a proponent of nested threads here ( I whined when we switched) I can do it by hand without difficulty if I find I need to, and those who don't know any better don't annoy us with needless nesting simply because they don't want to , or know they should, trim after they hit reply.
