Sometimes you over-roll and it affects the contact patch of the tyres. You'll also find that a car that is rolling will become more inconsistent at the limit because it is changing it's position quite a lot as it loses and regains grip. On high-grip surfaces you don't notice this phenomenon so much which is why cars can get away with softer settings.
I used to run a lot on slippy gym floors here in the UK with spiked tyres and very soft setups wouldn't work, the cars slide was more progressive and controllable with a harder setup. I've also seen it very clearly with the modern 4wd buggies which often handle better with a rear anti-roll bar on the very low grip tracks.
Basically there is a window of roll stiffness and geometry where the tyres are going to work, if you are either too soft or too stiff you are going to have problems.