well first he was fighting tobi so all the different rasengan wouldn't have really been effective. relying on his insane speed and strenght was the obviouly more effective way to attack untill u can get a clear shot on the intangible obito which is when he used the rasengan with success. why waste chakra? also beast bomb is now possible which wasnt the case at the beginnig.Ever since naruto got BM he been using brute force and Tbb's what ever happend to Mini rasengan:shrug:
Or is it that after the fight with Madara and when its time to fight sasuke is when hell pull em out again?
Maybe the fox gets sealed in order to stop the Moons eye plan and he would have to depend on Sage mode and all theese new rasengans to beat sasuke.
But i doubt he'd be able to use Rasengan Vacuum without the fox's power