Why don't people utilize the Rinnegan to its fullest? This question contains spoilers for anime viewers and early manga readers.

We've seen Nagato use his Rinnegan to the fullest, using the Rikudo technique (to control 6 bodies)


   As well as by fighting alone, as a reincarnated zombie.


He utilized all 6 paths of the Rinnegan.




   So why don't Obito and Madara? Madara only used the Preta Path, to absorb incoming ninjutsu. Obito only tried to use the Human Path, to try and kill Naruto.


Why don't they use any other realms? Their Rinnegan is supposedly perfect. I can't imagine they're holding back on purpose, the Rinnegan has some pretty scary abilities.
We can be fairly certain Madara is just too strong without using the Rinnegan at all. Although Madara's strength has never been explicitly discussed, we can infer it indirectly through other plot points. Madara fought on par with the Senju Hashirama, who used to have the 9 bijuu as his "pets". Other badass characters have had a lot of trouble dealing with just one. 


   It is heavily implied that the battle happened before Madara activated his Rinnegan. During the fourth ninja world war, Madara effortlessly mopped the floor with the five Kages. If Madara did use the Rinnegan, Kishimoto-sensei may have chosen to skip it to retain suspense over its powers.

Obito has only implanted the Rinnegan (and not "earned" it, like Madara). Maybe overusing Rinnegan drains his chakra rapidly (just like overusing Sharingan does to Kakashi), and hence he chose to not use it fully. Alternately, Kishimoto-sensei wants to save it for the climax.


Nagato being able to use it could be because:


   Being from the Uzumaki clan, he naturally has a huge amount of chakra. Madara likely chose him to receive the Rinnegan for that reason. Madara's plan involved getting himself revived through Rinne Tensei. He had to pick someone who could reliably use the Outer Path without passing out. 


Another key difference between the three:


   Nagato received his Rinnegan when he was a child, and must have learnt to use the full powers of Rinnegan through years of practice. Madara awakened the Rinnegan shortly before he died, and Obito has had it for only a few weeks at most.