So often times people will wait in a lobby to see if someone leaves or fails to ready up and gets booted so they can have that slot that they want which is already taken (e.g. sniper and spy slots are very popular for HL).
It would be cool if you could say pick a few slots as a backup, and if someone fails to ready up then you automatically get their slot. So for example you see a highlander lobby which has 17/18 players. You want to play either spy, sniper or medic, so you click on those classes and hit a button that says backup or something. The lobby fills and the sniper on red team fails to ready up, but you are there and selected that you are willing to backup his slot. So you instantly fill his slot and everyone's happy and the lobby gets started faster.