It seems I have failed to provide understandable explanation of what and why I have proposed. Let me try to reformulate it.
Random targetting leads to the effect we discuss here only in combination with an ability to "change lines" during combat. You see masses of enemy infantry and/or light vehicles - you bring your infantry and AP vehicles. You see "naked" armored machines - you bring your AV weapons.
What I proposed - is to effectively take away this ability to change lines. During subphase fights player will have no ability to change front line forces. Iterative fights during long combat phase will bring a necessity to keep every frontline formation mixed, because if your formation will have only AP weapons - it will win first subphases and fail against armored core of enemy formation in the end, and if your formation will have only AV weapons - it'll fail at the beginning. Bring random division of local battle groups - and you'll have much more dispersion of probable results, so lesser temptation of micro-calculating even if you have full knowledge of enemy forces (though this calculation will be available for those, who can do such things, but it will give them lesser advantage per hour of math work, and they'll have no obvious, easy, boring solution from the first glance, that we are obliged to close our eyes at, if we have to play with more interesting formations). This semi-random division of locations and iterations of fights will also bring deeper, however nearly hidden battle stories at the background - we'll know why we must send mixed forces, we'll have no need to close our eyes an envision things that will contradict with game screen. It will be just more RP-ish battle mechanics per se.