Why would it not be stronger with the grinding? Against a human opponent, I can see it being not as effective, but that kind of force is already within the Rasengan, it doesn't need to build up to be blasted into the enemy, unlike the Rotation, which grows in power as its formed. As for the Rasengan's...well, just look at its clash against the Chidori. The Chidori should logically pierce straight through the Rasengan's rotating chakra if it had no force pulling it back and preventing it from doing so. Yet, not matter how many times the two jutsu clash, the Chidori is prevented from getting through it because the blast of its clashing jutsu, which pushes it back as soon as they clash.