Proper rain driving (with actual wet line, even without puddles and such) would also be probably impossible on layouts, as the game would need to know where cars are actually driving to "get the rubber in" (or at least it would be much more difficult to have that on concrete parts), but don't get me wrong, I'd love to have rain in LFS, just later down the line.
Simply lowering overall grip wouldn't be very interesting, even if it would at least allow people to risk not using wet tyres (sorry michal). And a dynamic track system (temperature varying with time of day weather, and cars driving on it, and bonus points for dirt/debris/marbles) would very likely be needed before rain is even considered.
