They still can for paper tickets. Most of them don't want to though, unless its a expensive ticket. (example would be NW's GO BUMP program)
Part of this I suspect is a lack of ticket agent training in the way airlines get reimbursed.
Originally Posted by TrayflowInUK
When I was young, I remember my mother explaining things like, "Your father has an Eastern Airlines ticket, but he can go over to United and use it there,"