I think OUPT was implimented because of lazy math. In hardcore strategy games like Hearts of Iron or War in the Pacific, groups of mixed units are pitted against each other simulataneous using all sorts of variables that can affect the outcome of a battle. The Civ I-IV "stack of doom" was inheriently flawed because combat for each unit would occur successively and with relatively simplistic math. When they switched from stacks to single units, they made the mistake of not redefiniing combat units to focus on their purpose. A real army is comprised of multiple types of units (the stack of doom being a over-simplified version) that work together for a certain purpose and you lose a bit of that with OUPT. I do actually prefer OUPT, but the current implimentation is weak.
In hardcore strategy there is a concept called "frontage" that limits your ability to stack units ad nauseum. There is only so much geographic space in front of a unit, therefore there is a physical limit to the amount of troops and/or hardware you can attack with at any given time. OUPT deals with this, except in the case of Civ V the tiles are too big and it dumbs down actual strategy. My opinion is that they should split the exist tile size into 4 pieces and keep OUPT. In the place of one current tile, you would have space for four units, such as one armored, one mech inf, one artillary, and one support or whatever mix you want. Choosing how to mix my units would be so much more satisfying than what we have now.