Like it or not, Jobs' is intrinsically linked to the image that is Apple. If he were to die tomorrow, the stock would nosedive. Would they still be able to create the products that they do without him? Would their products still be seen as elite (read: retarded) if he's no longer there? Those are the kind of things shareholders worry about, and being that Apple is a tech company, it's all relevant here.
If you ask me, the cancer is finally getting to him, and this is a sly transition of power. It might lessen the shockwave if Jobs gives up power to the man who has already been running the company for 6 months.