They did the same thing with Jackson after the first Iron Man. They won't necessarily use said person in nine films, but its always good to cover your bases and have the option, seeing as all the films crossover when possible. A year after Avengers, we have Thor 2 and Iron Man 3. The year after is still being worked out, but we'll likely see another Hulk, Captain America, Avengers sequel as well as a SHIELD spin-off and a plethora of smaller budgeted second tier character-based films, so there are a lot of options for these extended contracts.