Listen to Niki, he knows what he's talking about.
Although I got used to it, I too felt theres too much grip loss when tyres get dirty. And after experiencing it in S2 now it gives me creeps every time I see a car put a wheel on grass in any other sim/game or when watching real races.

For example in last BTCC race at Croft cars were often taking a lot of dirt on all 4 tyres cutting the flat right kink before the very fast right corner, and I was concerened whether they would loose it there, but no, they just kept on racing hard and close as ever. Of course I don't know what drivers felt at those times but from the TV cam I couldn't see them slowing any bit at all. I know if that was LFS I would have to slow down quite a bit or slide off the track.
The section after back straight in Blackwood can be a real disaster if I put a left rear wheel on the dirt on the exit and then everybody else catches me through the next right S turn when I have to take it hmm 10-20kph or so slower. In GTi.