see that’s the thing. Back in the uneducated age, we had bi-amping. That’s where there are two separate amplifiers inside the head, with an adjustable crossover, so the one amp handled high frequencies, and the other handled low frequencies. In this scenario the 4x10 would get the highs, and the 15 would get the lows. Your job then, was simply to find the best crossover frequency, then balance between the two outputs for a good overall sound. In this scenario, much like Tri-amp PA systems, it made perfect sense to have different speaker arrays for highs and lows. The trouble started when people started pumping the full range amp signal through both cabinets. That was when you were subject to the potential ill effects of mismatched frequency responses, power handling capabilities, and phasing issues resulting from overlapping frequency dispersion to multiple partners.
Plus, as has been mentioned before, it just looked really cool behind you.