In reality, idling is the lowest rpm your car is running at so it would cause the least damage regardless whether cold or warm. Oil temp is the main cause for any possible damage and idling is the fastest way to raise oil temp without raising rpms. No matter how slow you drive, the car will not run at any lower rpms than if you are idling.
The only advantage over idling that driving slow really gives you in this regard is fuel economy. That said, if you are pretty careful, the difference is probably not that significant...it is just not true that idling causes more damage to a cold engine than driving slow.
I idle my car for this reason unless I'm in some serious hurry (which is pretty rare), but I can understand why someone would prefer to save some fuel and drive their car slow.
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