as seen here and here , it seems that sarutobi was worried about the third being summoned and I always wondered why. people say that it is because he was just way too strong and probably could not have been defeated, but i have a different theory to put alongside that claim. Minato sealed the kyuubi, or at least half of it in himself (the other half going into Naruto), and then died. The half he sealed within himself is the 'taking' half not the 'giving' half, and if he were resurrected and his soul brought back over, it would have brought back a very dangerous soul with his. The 'taking' part of the kyuubi would come back and then maybe be able to break free if Naruto's 'giving half was around,even if it was just at half power it would mean not only an enormously powerful shinobi but the strongest of the bijuu would be upon konoha.