The rematch clause is meaningless and only comes out into play when WWE need a heel to get some heat or a reason for a face to have another match for the title.
As for ADR, given the rumours that he's into his "politics", one could discern that he wanted back in a world title programme and wasn't happy to just work the upper- midcard/main event. Since CM Punk and Chris Jericho are involved in the WWE Championship picture, the next available spot would be the WHC - since WWE seems to view Daniel Bryan as flexibile. Of course, it could all be down to WWE solely and wanting to give Del Rio a boost in the momentum department.
Let's re-review the apparent pecking order of full time performers in the WWE:
John Cena - This has been his position since 2010
CM Punk - Punk's current run with the WWE Championship puts him here
Chris Jericho - WWE Championship number one contender status puts him here
Sheamus - His status as World Heavyweight Champion puts him here
Randy Orton - His status as the "face" of Smackdown puts him here
Alberto Del Rio - His status as WHC number one contender puts him here
Daniel Bryan - His position could drop dramatically at anytime, but for now his status as the number one heel on Smackdown keeps him on the pecking order.
As you can see, it's a very temperamental list with only John Cena being a fixture.