Well the username used to be stored in the post.
So you wrote "Posted by JoshPet:" into the new posts quote block.
But now..... when I change my name, it makes no sense.
You you'd have to store the number of the person you are replying to. Re: User# 2015, then do other queries to have it look up the current display name of user 2015.
I don't understand it completely. But rick says it requires a second JOIN.
If the Re: Username issue gets worked out... then the name in the autoquote would be easy. But currently, storing the name would make it extra confusing if they changed their name.