I do agree with you on that one. I think once a driver has progressed into full-season at the cup level, he should not be allowed to drive at the lower level. It's like having a baseball player in the major leagues also playing in the minor. Just doesn't sit right with me.
From a marketing viewpoint, these lower level races do not draw the crowds like the Sprint Cup does. To attract crowds to these races, NASCAR allows drivers from the top level series to race in hope......to draw a slightly larger crowd in the stands and to have more people at home watching these racers. It also helps out sponsors who cannot pay the large amounts of money for the Sprint Cup series drivers to get some TV time with the lower level drivers.