I think I know why Hendricks is fighting at UFC 158 after insisting that he would wait for a title shot.
It's simple. Johny knows (or was told by DFW) that there is a chance that Diaz will screw up and not make the fight. So if that happens, Johny will be able to slide into the title fight. And if by some chance GSP gets injured, Johny can fight Diaz for the title shot that's rightfully his.
For the promotion, it's great insurance against another UFC 151 type of situation, and for Johny, it's a chance at getting a title shot sooner than he may have thought.