I used this feature to good effect on a work trip last year; a co-worker and I were going to a conference, and we had to get the tickets through the approved travel agent. So we got E-tickets and made sure they had our FF#s on the record; by the time we got off the phone with the agent they were up on delta.com and we could fix our seats.

(In fact, the itinerary that the TA faxed over had the seats we picked out on it, I guess we changed them before they printed it out... )