The point is that the way YQ is used by carriers has changed completely. It's no longer an add-on, it's now basically a fare component and from the passenger's perspective functionally no different from the component labelled "airfare." (Again, excepting miles redemptions on certain carriers which charge it separately on an award ticket.) If you're familiar with how international fares work, you're familiar with how it's used. As WineCountryUA already explained, it's particularly useful to the airlines because they can change the price of the ticket without filing a completely new fare.