IDs at least instill some sense of order. Otherwise discussions tend to descend into 'samefag' self-promotion matches..
e.g.

6 "You suck"

7 ">>6 You tell him, this guys a genius!!!"

IT helps decipher genuine community support and someone trying to make themselves look good. It's also hilarious when users forget about the ID and reply to their own post 5 times.. :D
Sure, people can still use proxies- but it's too much effort.