I've spoken to Air Traffic Controllers about this during a pilot information night. Every one of them I asked said they had no problems with properly trained RAA pilots using controlled airspace. When I told them about the time I got stuck for half a day waiting for clouds to clear in class G, where there was a nice clear path through a bit of class C, they were amazed that I didn't explain myself over the radio and (illegally!) get a clearance.
If the controllers themselves don't have a problem, why do some GA pilots?