Part of the 'cost' is interference - you know what happens when two transmitters open on the same frequency at the same time. There were some neat technical tricks played up in northern Scotland at one point - don't know if it still happens - to get good low level coverage for helis. 'Repeater' transmitters were offset from the nominal carrier by 5KHz but last I heard aircraft kit designed to work with 8.33KHz-spaced channels would probably not work with such arrangements.
Just a coiple of thoughts from a non-technical person - no doubt engineering colleagues can give a fuller answer and correct mine!