Due to larger and larger bags being brought on as carry ons, the FAA and the airlines have been looking for away to police this. The little box at the gate allows to little control over billing for additional bags. Plans I have heard being thrown around are to have it done at Securty check points, so that if the bag is too big you must go back to the counter, check, and pay for it. The gate check "because I'm stupid" loop hole will be closed.