As the topic says, the button is unnecessary as the "Reply to Private Message" is all needed there and works as expected.
"Adv Reply" button leads to an error page showing-
The link on the button is : http://ubuntuforums.org/newreply.php?p=&noquote=1 which doesn't make sense IMO.Invalid Thread specified. If you followed a valid link, please notify the administrator
Sorry if it has been discussed earlier and it is a problem to remove it.
Also, obviously it is nothing too important IMO, but may be just a bit confusing for the new users. So just a heads up