I drive Pan cars 1/8 2WD, we do use differential.
What I find is the harder you set the diff, the faster your car becomes. Having a rather "free" diff brings you OVERSTEER ( not underseer ) getting into the corner off-throttle, cause a free diff can't act against yaw. And UNDERSTEER going out, since the inner weel will spin, hence the engine screams and doesn't push. No push -> no steering.
We can't use a spool because in 2WD we don't have the front pulling out, so the car would spin once the engine hits torque.
Another foundamental issue brought by differential is BRAKE.
You are aware that pushing hard on the throttle makes the inner weel spin and the engine scream, right? Well, brakes make the same, but in reverse!
I mean, if you brake with the inner weel lifted, the outer looses brake and the inner spins backwards.
So, compared with 4WD&spool, we have to brake a lot before and getting into the corner from a wider line, so you pull the brake with both the tires on the ground, maximizing the braking force.
Then, as they said before, you've got weight...... more than 100g in my case...... think about the lightness of a modern spool.......