Center diffs are used just the same as the front and rear. The front and rear diffs are needed because during a turn the wheels on each side of the car are turning a different speed.
The front tires spin at a different speed than the rear. But not as much of a difference as side to side. That is why in real cars and diesel trucks the center diff is real small. In a 4wd car the front tires are forced to spin the same speed as the rear. The car does scrub some because of this.
The problem is the insane accelleration and power of these cars. An open center diff would just unload to the front and the front tires would be spinning excessively on power.
An AWD car has a center diff and would have more turnin and less steering on exit. It would be nice for turning, but not for straight line speed. It would have to be a very good LSD diff to work. I thought the new LSD diffs would be too stiif and cause offroad cars to be too loose. With a proper setup they engage slow so that they don't lock up until your straight and that helps straight line acceleration.
Then again its another complex and heavy part. If the cars desighn works without it, its better. If one could be made that engages smoothly so that it deosn't start to lock up on turn exit, but is loose in turns and super tight on the straight it could work.
I was abit surpized when I found out that onroad cars were 4WD and offroad were AWD. Go figure.
Is it really needed.