Another problem will be to determine whether to charge the person $15 or $25 at the gate. If the person already checked one bag, then the fee for the gate-check should be $25. If no other bags were checked, then the fee is only $15. What a mess, as was said.
I am still amazed at how many very small pieces of luggage come off the checked luggage carousel at baggage claim. My guess is that half of these will be carried on if the passenger will be charged for them. Thus, much more carry on luggage in the same finite amount of room for carry on luggage.
I don't know, I'm sure they could just have the number of bags checked in the computer - wouldn't take more than a few seconds to figure out the fee. As for the small pieces of luggage, you're right to some extent, but I also bet a lot of them are toiletry kits filled with liquids that can't be carried on.