I believe the idle cable was put there for liability reasons. Mostly to cover up sloppy maintenance issues.
As far as it's function you can make an argument both ways, but it works much better in theory then in practice.
The pull cable opens the throttle against the force of the throttle spring, and on a single cable system the spring closes the throttle.
The two cable system works exactly the same way and the idle cable has no use (other then in Cruise Control bikes), except that if the throttle were to stick the idle cable would mechanically close the throttle by turning the grip with your hand, as opposed to snap back under spring pressure.
If the pull cable just breaks the throttle snaps shut by the force of the spring.
The idle cable is most useful if the throttle spring were to break. It is also useful in not properly maintained bikes where the grip itself does not move free so as to not allow the throttle to snap back on it's own. This is sadly very prevalent with Harley's as very few bother to optimize this sort of thing. It is also often the nature of the beast with aftermarket bars or grips not properly clearanced.
The downside of the two cable system is that it makes adjustment more critical, doubles the number of components that could cause a problem, and most importantly it has been my experience that in the typical failure mode where a cable comes apart internally the idle cable is usually not strong enough to return the system to idle and you would have been much better off to go for the kill switch to begin with then waste a second or so on something that does not work anyway. The idle cable could also develop an issue and then you would have nothing.
Internal throttles are quite finicky and mostly used on custom (show) bikes or TT bikes (tavern to tavern), not touring bikes. They are run inside the bars together with the electrical wiring, but these bikes have minimum wiring, much less wires then a touring bike.
It is a looks thing not a reliability or performance or function thing.
It can be done but you will have long term reliability issues, if you are thinking touring bike you are pi$$ing into the wind. The fairings hide most of this stuff anyway.