My Uncle drove a government issued Jeep in Europe
You are assuming we are interested in obstacles. I rarely attempt "obstacles". I find lockers, front and rear, useful over the road too.A rear locker is 90% of it. A front adds only a little. But you need the experience seeing different rigs and different drivers succeed or fail over many different obstacles to see.
With lockers, if one tire is on ice, the other 3 are still pulling.
Without lockers as soon as 1 tire spins, the other tire gets no power and you now have 2 wheel drive.
I take it by obstacles you think lockers are only useful on granite.