Why use BUIS, why not BDC irons as primary? Think about it, on E type targets, BDC irons allow for hits at UKD to about 400 meters with BSZ; and, using BDC function, the sight can be hastily adjusted for KD hits out to about 600 meters. And, the only thing irons require for good hits are the shooter's recognition for the need to properly point the rifle with consistent sight alignment, and focus on the front sight. Those tasks are easy to accommodate since the eye wants to naturally balance and center things.
Thing is, most folks simply do not know how to properly point the rifle with irons; and. therefore, not hitting where aimed with such sights, the shooter considers them inferior to optics. The fact is a trained marksman has no need for an optic in most scenerios where a relationship between sight and target can be established. Such relationships can clearly be established out to about 600 meters when shooting at something like the E type target. Still, shooters buy optics thinking they will get better results than when using irons, and although the optlic will certainly serve as a substitute for marksmanship it will also as surely deceive the shooter into believing he actually knows how to shoot when in fact he knows nothing about good shooting, as evidenced by his inability to get good groupings with the scope on anything beyond mid range.