As far as I know its impossible at this time to say how many Legionaries used the armor, or who specifically used it. There is some evidence that Legionaries were required to buy their own kit (weapons, armor, and accoutrements), the money being deducted from their pay. If this is true, then the Legionary may have had to choose between the Hamata, Squamata, and Segmentata. Maybe, because the Segmentata was easier to make, it was cheaper to buy, and therefore some penny pinching Legionaries bought it for that reason. Who knows? We can theorize all day but we will only know for sure when definite evidence is found, until then.......