I have a feeling (and this is not scientific at all, just based on what I've read here) that if one were actually at the airport the previous day trying to standby and a seat was truly available, that many TAs would allow this as an override. I can see why a phone agent might not as the load on the flight may still be in flux. Closer to flight time, I would expect more leeway...but it would involve a (possibly futile) trip to the airport and an understanding TA (possibly GA if one can get a priority card or something to get past security).
Cheers.
I suspect you are right. That's why I just went to CDG instead of calling - granted, I had to fly out that day so I would have paid the fees if I had to - but at the airport there was no issue nor was I asked for a reason.
I'm guessing that if it became "standard" to confirm it over the phone people could start abusing it. If someone is actually at the airport (expecially high elite levels) they can be more flexible w/o setting precedent.