This is true that left to right differential action is still in play, however, the center differential exists because there is a need to differentiate the rotational speed between front and rear as well. If there was no need for this, manufacturers would not add the complexity and the cost. You need to consider there are different traction issues in play here. For purely forward traction under power, CDL locked will offer superior performance. Aside from just being stuck, does anyone actually get into trouble with a lack of forward traction? I guess if you do, you're just doing it wrong. Where people get into trouble is the loss of lateral traction. As steering inputs increase, there is a greater rotational speed differential between the front and the rear. Since the vehicle is, for all intents and purposes, a rigid structure if the wheels need to turn at different rates, but are locked together, something must give (this is physics and not subject to opinion). That something will be traction of a tire. Once a tire has lost traction in a corner, it loses much of its ability to resist lateral loads this leaves only 1 tire available to keep a given end of the vehicle from sliding sideways.
Driving with the CDL on does feel more responsive because when you feed it the skinny pedal you have an additional tire pulling, but that feeling does not actually translate into better handling performance.
