Despite the fact that Tousen is unaffected by Aizen's powers, there's really nothing from stopping Aizen from just beating the shit out of the blind guy whenever he feels like. If Tousen defects, he'd better have some really good abilities/skills/tricks hiding somewhere.
And I'm agreeing with Outlawz on the point of Noitora. I personally think he's an amazingly powerful character with loads of potential, not just some guy that's there to act sleazy whenever he pops up. If anything, he really just needs to be delved into a bit more, which is what I'm hoping will happen once Szayel goes away again.
Not to say that Szayel's not amazing, either… I'm also hoping for a fight between him and Mayuri someday.