Besides the fact that she's a main character and it wouldn't make sense to put it in anyone else but her...blahblahblah... I'm really curious to know why. And HOW did he do it? If there are only those two ways of extracting the Hougyoku from a soul (destroying the soul with the Soukyoku or by using that green arm thing), then how did Urahara PUT that dang thing in Rukia's soul in the first place? Was it in the gigai that he gave her? Theories, anyone? I can't think of a decent explanation. I can't see Byakuya knowingly letting Urahara do something like that to his precious sister.
And after watching that scene again, I can't help but feel this kind of "jealousy" (for lack of a better word) from Aizen towards Urahara. I'm beginning to picture the two of them back during their academy days with Urahara being the oh-so-cool-but-incredibly-smart scientist with all these great inventions and Aizen with not-so-great inventions of his own. And what if Aizen had something to do with Urahara being exiled? Yeah, creating a gigai that drains a shinigami's powers is bad, but I'm sure Mayuri can create things that can do way more worse than that.
While I'm still on the topic of Aizen, has is ever been revealed as to why he's doing all this? I know he wants to create a key, so he can enter the realm that the King is in and destroy him, but WHY?
Anyway, I'm just being a Bleach fanatic right now instead of studying. Please share your thoughts; all these theories are running around my brain and there isn't anywhere else where I can set them free except here.
By the way debbiechan, please feel free to delete this post if it's not allowed.