I find it interesting the Kabuto intended to use Nagato to kill Killerbee and Naruto temporarily and then revive them. So killing and reviving people by removing and then returning their souls seems to be something that can be done without too much difficulty. I have to assume that it is more complicated than that when it comes to truly resurrecting someone who has been killed by other means (as Tobi mentioned, he seemed quite irritated that Nagato wasted the power on someone other than himself).
http://dl01.mangashare.com/manga/Naruto/551/009.jpg
As POW mentioned perhaps it is a case of their being various levels of revival and various levels require a more substantial amount of life force.
Yet I agree with the basic argument that there must be some limitations on Nagato's ability to resurrect people or he more than likely would have just resurrected Yahiko and anyone else that was of importance to him. Having a body might be necessary but I kind of doubt that is the only limiting factor. Tobi has made a lot of criptic remarks about the technique and the very idea of using the technique for himself seems odd. I am guessing that Tobi (who has constantly refered to himself as Madara) was infact refering to bringing Madara back to life. If it were as simple as Kabuto suggested for Killerbee and Naruto to be resurrected though, I have to wonder why he hadn't done it already. We still don't know how Madara was killed, or what Tobi really intends to do, so I am guessing we will learn more about the technique when that information comes into play.
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