I'm pretty sure if Shuichi is killed, Yoko would be able to finally move on from the body so there is a threat there. Yoko may not need Shuichi to live, but they at one time did need one another. But I feel right now, they're on pretty damn good terms and Yoko isn't in a huge hurry to move on since he's become fond of Shuichi and his mother. And I do think even as a spirit, she may weaken if she has to take a new physical form, maybe to point where her spirit is taxed beyond future use. I'd like her to have some mortality/finality, even if it's not as pressing as a human's. 

So yeah seems like we're on the same page. -nods nods-