The copper situation is ok in inner city areas (except some buildings where the wiring is corroded, or the two wires split to different lines which increases noise/slows the connection), but ADSL speeds also depend on distance. Many areas can't even get into an exchange because they're full, and when you do if you're too far away from it your speeds will be quite horrible.
Not to mention upload speeds, NBN would make it possible to send virtual images, giant movie files, HD video and other big chunks of data person to person, rather than this many to one subscriber download model we kind of have at the moment.