Actually, it's about lubrication and letting all those aluminum parts expand and reach their "happy place". These air cooled engines are set up with large cold clearances and low clamp loads on things like head bolts because of the large amount of expansion they see from cold to fully hot. It is best to let the bike run until the rocker boxes are warm to the touch (about the amount of time it takes me to put on my jacket, helmet, and gloves). And then it is wise to drive conservatively until the engine and oil temps stabilize. I don't use a temp gauge, I gauge it by the oil pressure. When the oil is below normal operating temp the pressure runs higher. When the idle oil pressure drops to the normal 10 psi, I figure it's up to temp.
Another point, long warmups at idle are not needed and may in fact be counterproductive. The engine will warm up quicker and more evenly if it's under load. This is true no matter what kind of engine you're talking about. Long warmups at idle basically waste a lot of fuel, some of which is probably washing down the cylinder walls if you have a manual enricher system.
Jerry