Tires are not rigid and flex, this helps turning as the contact patch deforms to allow the change in direction.
We have a term for what Paul is describing. FLAT SLIDE and in sprint karting not desirable.
TRACTION CIRCLE. A tire can only give you 100% of one thing. You can only ask a tire to give you 100% acceleration, 100% deceleration, 100% steering. Once you ask for it to turn and slow or accelerated you will only get a reduction in overall performance.
Tire Friction. Once a tire advances over the threshold of 100% performance it starts to slide. To stop the slide you can go back to 100% and still be unable to achieve friction. To have grip you must go well beyond 100% to somewhere in the area of 85% to allow the tire to again build friction and grip.
So therefore a setup set to allow the rear tires to slide is a reduction of overall performance of the tire and will cause a reduction in deceleration and acceleration available from the tire.
Not to mention I sure as hell don’t want to be in a laydown position trying to stop my body from going forward when braking with a kart with full brake system, or dealing with the turn in forces, or even better the curb hoping.