I did stand-by on my last two DFW-AUS flights and, despite being Gold and near the top of the stand-by list, ended up with middle seats, far in the back. The same happenned on a LAX-DFW flight last year, despite quite a number of open seats. MD-80 far in the back is rather loud!

So, I do think agents could exercise a bit more judgment (at least if there is time) in assigning standby seats.