To answer your question. Remember when Kushina stated that she would take the fox down with her? Well the fox would restore itself, creat itself from nothing to something. Half the fox power was sealed inside Naruto, but it would one day gain full power back once it is fully released again, just like Killer Bee does when he transforms. So once Naruto learns how to completly let the ox out, he will be able to gain it's full power.
Naruto locked the fox away, but the seal used to do so allows him to take chakra when he wants. The fox has no control rather or not if he wants to give Naruto chakra anymore. --- PSN: Silver_ShadoWolf GT: ShadoGhostNinja
Like Minato said, he couldn't possibly seal the full power of the Kyuubi into Naruto. If that's because he's too young or that the seal Minato can use isn't strong enough, we don't know. It may also be that Naruto doesn't have the special chakra Kushina and Mito had (as we can see, Kushina's chakra was insanely strong - being able to restrain the Kyuubi's full manifested power while she was moments away from death) thus making controlling all the chakra an impossibility for Naruto.
As for why he had to seal the body of the Kyuubi - it's probably because the Kyuubi is still filled with hate and malice towards Naruto, while the Hachibi within Killer Bee is on better terms with his host. --- If you want to make an apple pie from scratch, you must first create the universe.
It's not so much control - it's that he can't be 'friends' with it. Killer Bee gets perks from his Bijuu (like having him snap Killer Bee out of genjutsu) simply because the Hachibi likes Killer Bee. Unless Naruto gets the fox to like him, he'll never have that kinda relationship with his Bijuu. --- If you want to make an apple pie from scratch, you must first create the universe.