You are correct.
Off road, the brake based systems tend to at least require programming changes to allow some spin, to "get going".
What happens off road for example on systems that have poor programming, is that the truck will just sit there shuddering. Essentially, as soon as some torque is applied, the system senses slippage and brakes the spinning tire....and if none of the tires has enough traction on its own to make the rig MOVE, well, the poor things sit there shivering and shuddering as each tire tries to rotate, goes faster than another tire, and is promptly braked.
By allowing some initial spin/rotation to occur, it also allows the other tires to try to pitch in on the propulsion thing, so some momentum can be gained.
This is also why, from a propulsion stand point, the front lockers can outperform the rear lockers.
Essentially, the locker really only helps if one side slips, by allowing the OTHER side to keep pushing you up the hill, etc.
The tires that slip the MOST though are the front tires, which are more lightly loaded, and tend to have less wheel travel to stay on the terrain.
So, lockers help the axle which slips the most, more than they help the axle that slips the least....and if it doesn't slip at all, the locker is not a factor.
Of course, its a lot harder to STEER a locked front end.