Ageless stranger has presented a good point. I mean what If Orihime could be used as the key. that would give Aizen a reason to want her. As for the fact that Orihime wouldn't join him... lets not forget what a mind job he did on Hinamori, plus the fact that he MAY have completley decieved Tousen's great sense of justice. Aizen is a master manipulator, he could probably make Orihime strip down and dance like a chicken if he wanted to.