My understanding is that the "mixture" is actually set by the Slow Speed Jet. All the "Idle Mixture Screw" does is set the volume of mixture, which is not as important to the big picture. 

The more important part for smooth idle is to sync the butterfly openings ("Idle Speed Screw" balance), and then for best throttle response (just above idle) is to balance the cable pulls. 

Above about 2500 RPM the slides take over. The slides are vacuum controlled, that is, they are independently responding to what the engine wants and thus, in effect, self balancing. That's why you want CV carbs on a touring/road bike, but Mikuni-type slide-only carbs on a sport/race bike. 

Makes sense. Thanks!

But if you don't get the air/fuel mixture screw dialed in correctly, you can get lousy fuel mileage, backfire off throttle, and/or fouled plugs.

Also important to adjust those after a 5-10 mile warm-up ride, or you get a 1500-2K rpm idle.