The only time you will ever have to pay at the gate is if you bag is oversized and should never have been brought on in the first place, or if you brought too much on. And that's only when you should be charged - I see this very rarely. Granted, I'm all for agents enforcing it more - if people know they won't get away with bringing more than they should and have to pay, they won't try in the first place. In a perfect world, at least.