I agree with you completely. We true wheels to within 1mm and all is fine. But we don't account for variation is tire manfacture and mounting. Tires are a composite of a fabric casing and wear surface (tread). I doubt the manufacture and installation of a tire is as exact as the trueness of a rim/spoke/hub. And once you throw in the variability of $3 tubes, meeting a 1mm overall tolerance cannot be guaranteed.
A true rim has a big effect on braking performance (assuming rim brakes).
An analogy would be the fallacy of improving my performance with light-weight components, titanium fasteners, drilling out and gram counting, but completely ignoring the fact that I am 15 lbs overweight.