The reason this match isn't Rhodes vs. Shield is because no one would figure the Rhodes boys would lose, and the company wants to insert an element of uncertainty in the proceedings. So, in come the Usos'.
This, and also, The Shield have lost twice to the Rhodes, and they're still hot enough an act that I believe WWE would be reticent to have them lose a third straight time in tag action to the Rhodes. For me, there's little doubt the Rhodes retain here, I just don't see them pinning the Shield.
Incorporating the Usos accomplishes a couple of things. One, it gets their tag title shot out of the way. The Usos had been built into a semi-hot act feuding with the Shield, but the Shield dropped the titles because of the hot angle with the Rhodes. But to WWE's credit, they didn't brush aside the Usos shot, so here we are, in a triple threat match. The Usos get their shot, likely take the pinfall, and they're back to being just another tag team along the lines of the PTP's or the Real Americans.
Second, this allows the Rhodes(if they do) to retain their titles without beating the Shield once again. While they're hot right not and the tag title switch was the right move, the team with the larger upside and the probability of staying together longer. Thus, having them lose to the Rhodes for the third time in 3 weeks would be a difficult sell. The Shield have been major players since they set foot in WWE, but their momentum has waned somewhat recently. Ambrose isn't even on the card, and as the 'personal security' for the Authority, having them being pinned once again could cause problems for their standing. There's plenty of mileage left in them as a trio, so having the Usos take the pinfall is the smart move. Or if WWE decides to switch the titles back, the Rhodes don't look like chumps so soon.
I'm going with the Rhodes to retain. They've got a great deal of momentum right now, and cutting their legs out already doesn't seem the best move. They won't have a 6 month reign like Team Hell No or The Shield, but it won't be less than 3 weeks, either.