I saw this happen with a COEX F/A a couple of days ago. Full out of MIA with several "mainline" passriders standing by. The agent had to offer the Express FA a seat before any mainline passriders! I asked the agent why and she said it was the "boarding priority". I had told the FA that the jumpseat was available and I'd appreciate it if she'd use it so I could get more passriders on. She seemed a bit "ticked", but took the JS................. Funny, but at Express, they ALL go on before me. ????????