I like what you all said, but I think the main thing it weaken Obito's resolve, and thus weaken his shield I think Obito gave up honestly, just look at his 'What-Ifs" it was all friend based. I think Kishi made it more of a symbol that defies logic and reason. Each of the Rasegans were massive, and there were 9 of them. A Bijuu bomb would do the same damage and more, but I think it takes time to gather the chakra, and they couldn't attack at the same time while the Bijuu bomb went off.