Pavel's add-on is not mandatory. In general I think it's best we all drive on the same track with the same solid objects. When I evade an object nobody else can see, I'm behaving unpredictable, which might lead to a crash. This problem wouldn't be there if everyone uses the same add-on.
Last line of rule 4.5 says "Only changes to optical or acoustical aspects of the game are allowed." As far as I know, the add-on that removes the pitwall of Kyalami is the only exception to this. I mean, at Kyalami you are free to use or not to use the pitwall add-on. I think that decision was made because the add-on contributes to driver's "safety".